Superb Documentary On Jazz Artist Chet Baker. Oscar nominee finally heads for theaters. FILM: REVIEW

`LET'S GET LOST,'' a documentary about the late jazz musician Chet Baker, was nominated in the recent Academy Award race and has played major film festivals, including New York's prestigious New Directors/ New Films series. Now it's finally heading for theaters: Its premi`ere engagement is in progress at New York's enterprising Film Forum, and its national release commences next month. And not a moment too soon. This is easily the best documentary, and the best all-around movie, I've seen in a long while - although its subject has sad and unsavory aspects that may disturb people with no special interest in jazz or biographical film.

Even by jazz-musician standards, the late Chet Baker was something of an odd man out. Check the notes to his currently available albums, and where you expect to see unvarnished hype you'll find excuses for his shortcomings. One speaks of his ``ups and downs'' and ``extremely uneven'' work; another mentions the ``somewhat erratic'' period it dates from; a third describes the ``roller-coaster extremes and musical paradoxes'' of his early career. With press agents like this, who needs critics?

Even his admirers found Baker a hard musician to pin down. During the 1950s, he built his reputation on the ``cool'' sound of the West Coast school.

Yet he also recorded with members of the hard-driving East Coast contingent, and didn't shrink from playing a Miles Davis number now and then - as if inviting, rather than repelling, the charge that he borrowed too heavily from Davis's patented style. On top of these musical matters, Baker led a tormented personal life, with unsuccessful marriages and persistent drug problems. He died last year while still in his 50s.

Why must attention be paid to such a man? The answer lies in his music, which was simply gorgeous. It's not to every taste, of course. His trumpet playing and singing are so unassuming - almost relentlessly so - that they veer closer to pop than to jazz at times. And there's no question that Baker was always at his best in the ballad form, giving some of his albums a consistency that's almost too much of a good thing. His most successful music was so exquisitely conceived and controlled, however, that it must have reflected a core of dignity and decency within the man himself. Such beauty can't have been an accident.

In addition to his musical accomplishments, Baker summed up much that was best and worst in the culture of his time. This was especially true during his rise to fame in the 1950s, when his moody handsomeness and relaxed demeanor made him an epitome of ``beat generation'' charm. Later he sank into a more laid-back and even lethargic manner related to the excesses that dogged him. Yet his music stayed uncannily smooth and expressive, as his sessions in the late '80s attest.

To film ``Let's Get Lost,'' director Bruce Weber and cinematographer Jeff Preiss took their crew on the road with Baker, accompanying him across the United States and to Europe during the last year of his life. The movie shows him in candid moments, in music sessions, and in interviews before the camera. Sometimes he struggles to remember and speak through some kind of mental fog; at other times, especially when he plays and sings, he's alert and engaging. But he's always fascinating, in a way that's cautionary and exemplary by turns.

Rounding out the portrait, the film includes interviews with Baker associates and clips from his occasional feature-film ventures, including a Robert Wagner-Natalie Wood epic based on his early life and a couple of productions in which he acted.

``Let's Get Lost'' is a movie drenched in atmosphere, reflecting both the '80s, when it was made, and the '50s, which were Baker's good (and bad) old days. It casts a welcome spotlight on the crystal clarity of Baker's best musical achievements, and for this alone it's a welcome arrival.