What happens to a jazz singer when she runs out of songbook?

Where to turn when the standards seem exactly that, and the glow fades from the old reliable torch songs?

Diana Krall, one of the most successful jazz singer-pianists of her generation, experienced ennui with the repertoire, and decided to grow her own.

On The Girl in the Other Room (Verve, **1/2 out of four stars), which arrives in stores today, the 39-year-old native of British Columbia leaves behind the show tunes and Tin Pan Alley evergreens that have been sung by every aspiring jazz chanteuse since Billie Holiday.

In their place, Krall offers six songs she cocreated with her new husband, the protean pop tunesmith Elvis Costello. The pieces send Krall into shadowy film noir, the kind of stuff you'd hear in a rundown, slightly sinister, Coen brothers piano bar. They require her to sing sprawling, Sondheim-style tales, about love heists in progress and existential quandaries. And they find her brooding about loss in the forthright, confessional language of singer-songwriterdom.

It's the first such experiment in Krall's career, and one of the rare instances when an established jazz singer has tried to transition from interpreter to originator. It's bold, because in current jazz, singers thrive when they stick to the canon, and suffer at the box office when they experiment.

Many in Krall's audience come to hear what she does with even the most worn-out standards. The familiarity of the songs helps forge a connection, and though she might eventually take "The Shadow of Your Smile" down streets it's never traveled, at least the starting point is a known entity.

There's little of that jazz-and-cocktails comfort zone on Other Room's originals.

Krall sings in front of her customary small ensemble, and surrounds her vocals with the elegant tinkling-piano accompaniments and gently speared guitar chords of classic torch. The environment is understated and placid, and at times seems intentionally smooth - as if Krall was using the default "jazz" sound to soften her downright challenging melodies. You have to listen to these songs a few times to discern, much less appreciate, their melodic contours.

The most straightforward theme is the title track, a broken-tempo waltz that follows a girl caught up in romantic obsession. Krall and Costello leave lots of open spaces in the story: Each verse ends with a single line of lyric, and an image that hangs poignantly in the air, in much the way the best phrases on Costello's 2003 orchestral-pop North did.

Because there's no clear refrain, or even a summary clincher, the task of filling in the blanks falls to Krall's vocal. She rises to the challenge by singing wearily, supplying a regret that's only hinted at in the text. Her coy restraint, and her casually improvised asides, are what make the piece riveting.

Costello wrote the lyrics, sometimes from lists of images and notes Krall gave him, and on the brooding "Abandoned Masquerade," he transformed those raw ideas into an unmistakable expression of personal resolve. As is his wont, he occasionally tells several stories at once. On "I've Changed My Address," the account of long-gone love is underscored by a tale about a different type of regret, at how the club where Krall got her start is now a sports bar.

It's an impressive compositional trick - a contrast between modern malaise and openly sentimental melodrama - and it gives "Address" a dimension missing from some jazz standards.

Not all the originals are as effective.

"Departure Bay" is a maudlin remembrance of Krall's mother, whose death after a battle with cancer was one catalyst for Krall's new direction. "Narrow Daylight," while nicely sung, sits in the same ponderous ballad tempo that predominates here, and its melody wanders, somewhat aimlessly.

Perhaps more troubling is that Krall doesn't go all the way and commit herself fully to original songs. Her ambitious originals sit next to several pleasant, almost lightweight, treatments of rock-era covers.

Krall's version of Joni Mitchell's "Black Crow" follows the original a little too closely. "Temptation" takes Tom Waits' gutter allure to heights of prissiness the author probably never imagined. And though Krall tramples the Bonnie Raitt hit "Love Me Like a Man," she atones at the piano when, in an inspired coda, she adds a Count Basie shout chorus.

Then there's Costello's "Almost Blue," a song that's been covered by singers from all corners of popular music.

This could have been another perfunctory bit of business, but Krall sings it with a glassy-eyed grace. She sounds deflated, as if she's been through the mill and can no longer soldier on with a stoic face. The vocal performance unfolds word by disillusioned word, and just when Krall sounds as if she's given up, she drops the faintest hint of "My Funny Valentine" into the music.

In that moment, the guiding spirit of the project becomes clear, and Krall's transition to the "other room" seems like a smart, and hopefully long-term, move. There she is, decorating a song written in the '80s with a Rodgers and Hart melody from the '40s, singing about the hurts and ambiguities of romance in a way that's profoundly modern - and still somehow timeless.