"You know," I tell Diana Krall, "a lot of people are interested in your marriage to Elvis Costello."

The world's most famous younger jazz singer ponders this information, slouches into the sofa, makes herself smaller. "I'm so surprised at the press, you know?" she says at last. "I'm so surprised at this whole - I just didn't expect it. I guess that's a good thing."

What, it's a good thing that people are curious about you and Elvis?

"No - no! A good thing that I don't think about it!"

We are in a suite at the Four Seasons in Beverly Hills, Calif. It's a balmy afternoon outside. There's a nice fruit plate on the coffee table. Krall is discussing what might be the most audacious album of her career, The Girl in the Other Room (to be released April 27 on Verve/Universal Music Canada). Much of the surprise comes from the six tracks she co-wrote with 49-year-old Costello, the veteran of Britain's early-'80s post-punk scene whom she quietly married last December.

And as long as we keep the conversation to the musical roots of that collaboration, everything's cool. I have been interviewing the 39-year-old singer from Nanaimo, B.C., at intervals for almost a decade, and her instincts have never been those of a diva or a fashion plate or a socialite. In her career, of course, she has played all those roles. But in her head she's a musician. She makes a living putting on shows. She has made a very good business of it. But if you want to ask her about show business, you won't get far.

Yes, she admits, she did appear on Vicki Gabereau's afternoon talk show with Costello in March. But she is horribly distressed when I tell her the Globe and Mail ran an account of their appearance under the headline, "The Krall and Costello show."

"Oh, I don't like that. 'The Krall and Costello show?' " This won't do. "The reason why we did it, that cannot be lost in all of this, is the benefit."

Costello and Krall were in Vancouver for a benefit concert with Elton John that wound up raising $810,000 for the leukemia/bone marrow transplant program at Vancouver General Hospital, where Krall's mother, Adella, was treated before she died of a rare form of bone cancer in 2002, at age 60. Battling cancer is the cause Krall holds dearest. The trauma of losing her mother led, in a roundabout way, to this new record. And while Krall and Costello share composing chores, the benefit concert was nearly the only thing that could get them to share a stage. "We're not going out and touring like" - she begins to name a hokey, old-time husband-and-wife act, then cuts herself off and glares a warning at me not to embarrass the old-timers by printing their names. "We have our own identity. And our own careers."

Gabereau landed the joint appearance because she is an old Krall family friend. But "then Elvis and I went on and Vicki introduced us by saying, 'Grammy Award-winning newlyweds!' And I went, 'Grammy Award-winning newlyweds?!' It's so so funny to hear that about your life - you're not aware of how you're perceived."

Right, then. So how did the Grammy Award-winning newlyweds make the new album? At first, they didn't. They weren't newlyweds, and when it all began, they weren't collaborating. At first there was only a sensitive woman racked with loss and groping for a way forward. It was May 2002 when Adella Krall finally succumbed to cancer. A month later, two of Krall's most cherished musical mentors, singer Rosemary Clooney and bassist Ray Brown, died within days of each other. What do you do when loss follows loss like the waves of some horrible storm? Like so many of us, Krall went home.

By August 2002 she'd bought a house on Vancouver Island, near Nanaimo. She holed up there with an old hi-fi and all the LPs she'd accumulated "from the time I was 16 to whenever they stopped making them." Among them were the scraps of jazz history any young student might gather as she tries to find her way in this wild and demanding music: "Lee Morgan, Fats Navarro, Carmen McRae. Billie Holiday in particular - stacks of Billie Holiday - and Jimmy Rowles."

Rowles used to accompany Holiday, one of the greatest jazz singers there ever was. Twenty years ago, Krall came down here to L.A., still just a kid eager to make it, not a singer but a pianist if she was anything at all, which she really wasn't yet. She was looking for lessons from a master, and gruff, rumpled old Rowles was just the guy. Krall stayed in L.A. for three years.

More recently, back on Vancouver Island, missing her mom and staring at album covers, she remembered how eager she'd been to escape. "And if I could knock on his door right now? At 520 North Bel Aire Dr., up in Burbank? And ask him what these chord changes were to this tune that he did? I would. I would do it."

Of course Krall didn't just listen to old jazz records, because like any child of the 1970s and 1980s she didn't just listen to jazz. The old Tom Waits albums took another spin on the hi-fi. Joni Mitchell too. Mitchell records like For The Roses and Hejira got her thinking about "so many things that are important - that are specifically Canadian - that I can't really talk to a lot of people about," she says. "What it's like to want to go away so badly to New York. Be with the cats. Go to Bradley's" - the lost, lamented Greenwich Village club where Krall and so many other young musicians used to sit at the bar and watch the world's best jazzmen play what was, by common accord, the best piano in town.

And then to come back, so many years later, and spin the Joni Mitchell records she left behind. "You know: just write what you know," she says. "And there's a lot here [on Vancouver Island] that's a lot more interesting to me now than when I went away."

Slowly, at first without much purpose, Krall started to do what she had not been famous for doing. The subtle and insightful interpreter of the great old tunes from the classic age of jazz and popular song, chestnuts like Let's Fall In Love, How Deep is the Ocean, Cry Me a River, began to write. In this she had, as she demurely phrases it, "the good fortune to collaborate with Elvis Costello." She wrote most of the melodies. For the lyrics, she'd write down ideas and phrases. He would take care of the rest. Sort of. The division between his contributions and hers is, of course, hardly clear.

There are an extra few bars of minor-key filigree at the end of the CD's title track, The Girl in the Other Room, that could not have come from anyone other than Costello. Krall cheerfully acknowledges the assist. "It sounds like him."

So what was it like working with Elvis Costello?

Krall chuckles. "What do you think? I don't know how this will transfer to the page - which I'm learning; an eyebrow raised is hard to put into print - but that's like asking how would I like to collaborate with Bob Dylan. Or Joni Mitchell."

And what does the final product sound like? Well, first of all, it sounds unabashedly like a jazz record. Six of the tunes are by Krall and Costello. Krall covers a Costello number from before they met, Almost Blue, and songs by Mitchell, Tom Waits and Mose Allison.

Jazz covers of rock and pop are at least as old as the Beatles, and any good jazz musician is alchemist enough to make raw material her own. At the piano, with a succession of formidable bassists and drummers, Krall turns Waits, Costello and her own lines into performances that even Jimmy Rowles might have liked. "When people say it's a departure, it still swings," she insists. "There's probably more piano playing on it than there's been since the first record."

But in between the piano solos, the lyrics - both to Krall's own tunes and to the ones she covers - are shot through with homecoming, loss, longing and gratitude. The turbulent emotion of "an intense two years," she says. "And it was emotionally cathartic for me."

The CD starts off light and eases into heavier territory. One tune, I've Changed My Address, is a tribute to Bradley's, which turned from jazz haven into a sports bar while Krall wasn't looking. Love Me Like a Man, a blues shouter that Bonnie Raitt adapted and covered before Krall got to it, is a raucous ode to the most indelicate pleasures of romance: it is, Krall announces with delight, "carnal."

The second half of the CD changes the mood delicately, with the most old-fashioned tune of the set: an amiable, mid-tempo ditty called I'm Pulling Through, which Billie Holiday recorded back in 1940. Before revealing her most convincing work as a songwriter, Krall reminds the listener that a canny choice of an old tune can reveal as much about a performer as anything new:

When I thought that hope was really gone

You showed me I was wrong

And you taught me how to carry on

Thanks for the lift in time and thanks for your song.

But the real artistic triumphs of The Girl in the Other Room don't come until near the end, with two extraordinarily delicate and powerful songs. Narrow Daylight is barely more than a sketch, its odd phrase lengths making it seem almost incomplete. Its middle lyrics are fleetingly mournful: Although deep down I wished it would rain/ washing away all the sadness and tears/ that will never fall so heavily again. But it opens and closes with something more gently defiant:

Narrow daylight entered my room

Shining hours were brief

Winter is over

Summer is near

Are we stronger than we believe?

The CD closes with Departure Bay. It's about the ferry dock at Nanaimo, where young Diana skipped flat stones across the still water, where she heard the seaplanes in the distance, and where once she waited impatiently to ship out and into the big world. Now? I just get home and then I leave again, she sings. It's long ago and far away. Suddenly the song is explicitly about her lost mother.

Last year we were laughing; we sang in church so beautifully/Now her perfume's on the bathroom counter/And I'm sitting in the back pew crying.

It would be a bleak way to end an album if it didn't come after a couplet from I'm Coming Through, still another Krall-Costello original: Only the love you gave to me will save me/I think she knew.

Three times during our interview, Krall reminds me she's 39 now. "Things change, you know? I'm very driven, but I also care very much about having balance and time with my family, with my husband and my father and my sister. That's important to me."

Still, duty calls. She will travel to promote The Girl in the Other Room with few breaks until December. The tour launches with a free concert at Toronto's Union Station on April 28, advancing to a succession of cross-country dates starting with Calgary (May 2-4). She is considering a painting by Lawren Harris, of the Group of Seven, as a backdrop for her stage set. The designers were looking at "some Italian," she says. "I said, 'Let's just look at what is here.' "

Truth be told, the hardest work - the work she almost cannot stand - is talking to me and all the other reporters lined up to ask her about the Krall and Costello show. Is it ever helpful to talk about this stuff with a visiting scribe, I ask forlornly? "No, it's a drag," she says, and she can't keep from laughing. "If you must know, I find it difficult because it's such an intuitive thing. The reason why I don't think about it is because I express it in the music."

She is far more eager to tell me she'll be touring with Bob Hurst, one of the finest bassists in her generation, than to answer anything I can think to ask about the glamorous life. When is she happiest? "Every night," on stage, she says without hesitation. "When I'm doing something I can feel, and that I can personally relate to. When I'm playing with the musicians I'm playing with. It's the best part."

Maclean's April 26, 2004