Hindsight isn't always 20/20. If it were, there'd be no need for Mosaic Records and its extraordinary jazz reissue series.

Since its beginnings several years ago, Mosaic has provided listeners with a much sharper view and appreciation of the works of numerous jazz musicians, the legendary (Thelonious Monk, Clifford Brown, Charles Mingus, Bud Powell, Gerry Mulligan) as well as the obscure (Tina Brooks, the Port of Harlem Jazzmen).

The label's partners, Michael Cuscuna and Charlie Lourie, have gone about this the old-fashioned way, by combining painstaking research with a great passion for the music. They've reexamined the nooks and crannies of the artists' careers, restored the aural quality of their music (old Blue Note recordings have been their primary source), located rare and often striking black and white photos from the period, compiled new discographies and commissioned informative critical essays. If the results aren't definitive, they're probably as close to it as we'll ever get.

Two of the latest Mosaic reissues are devoted to trumpeter Chet Baker and saxophonist Paul Desmond. The Baker set is a four-record box of his early '50s Pacific Jazz quartet sessions with pianist Bud Freeman (Mosaic MR4-122). Desmond's is a six-record box chronicling his fruitful 1959 to 1965 association with guitarist Jim Hall (Mosaic MR 6-120). Possibly more intriguing, though, are the sets devoted to lesser-known artists, pianist Herbie Nichols, saxophonist Ike Quebec and guitarist Teddy Bunn.

Herbie Nichols Indeed, the Nichols set is the biggest find of all, nearly doubling the amount of recorded material previously available by this immensely gifted and intelligent pianist. Nichols, who died in 1963 of leukemia at the age of 43, had tastes that ranged from Jelly Roll Morton to Barto'k, Stravinsky and Villa-Lobos. He was also a prolific composer, sorely overlooked by the public and by many of his peers during his short life.

He did have his admirers, though, and chief among them was Blue Note founder Alfred Lion. Shortly before his death in 1985, Lion told Cuscuna, "I hadn't been so excited about someone since I first heard {Thelonious} Monk ... I wanted to record everything he had, just like I did with Monk in '47. His stuff was so original and it swung."

Lion got his wish. In fact, Mosaic's "The Complete Blue Note Recordings of Herbie Nichols" (Mosaic MR5-118) seems nearly as much a tribute to Lion's foresight as Nichols' genius. It contains 30 selections from the mid-'50s, and save for a dazzling version of George Gershwin's "Mine," each one of them is an original piece. Sadly, only one of them -- "Lady Sings the Blues" -- is particularly well-known today.

If commercial success eluded him, Nichols never wanted for ideas. He once wrote, "Sometimes I find it hard to distinguish where my technique ends and inspiration begins. I get ideas from Modigliani, Roualt, van Gogh and a lot of other painters and sculptors. Rhythms and patterns seem to be endless and I find them in boxing, architecture, literature, vaudeville, the dancing art of Primus, Teddy Hale and Dunham. All the world's a stage for the jazz pundit."

Throughout this selection Nichols favors a light touch, almost dainty at times. Yet as Lion discovered (and as Roswell Rudd points out in his touching and thorough essay here), Nichols was capable of swinging on even the slowest number.

Like many of the classical composers he admired, his strength partly derived from his analytical and adventuresome nature and his remarkable ability to evoke specific moods through his music. Even if you were to strip away their tell-tale titles, there'd be no mistaking the inspiration that gave rise to tunes like "Chit Chatting," "House Party Starting," "Terpsichore" or "Blue Chopsticks," just as there'd be no mistaking his command of a broad range of jazz styles. On tune after tune in this anthology, Nichols not only has something specific on his mind, he conveys it with great clarity, emotion and technical finesse.

Ike Quebec "The Complete Blue Note 45 Sessions of Ike Quebec" (Mosaic MR3-121) possesses a different flavor altogether. A companion piece to an earlier Mosaic collection of the recordings Quebec made for Blue Note in the '40s, this three-record set jumps forward to 1959 and contains all of the 45 rpm singles Quebec made for Blue Note from then until the last session in 1962.

What makes this collection so interesting is that Quebec, who always favored an earthy approach to the tenor sax, hadn't recorded since 1952. Hard bop and cool jazz had displaced his more soulful sound.

The new Mosaic set, however, finds him once again playing with a big tone, his phrasing more persuasive (and more romantic) than ever. Clearly, when the market for rhythm and blues recordings reopened the studio doors to him, Quebec was ready.

The mood here is warm and relaxed; the tunes, obviously chosen to attract jukebox play and radio airplay, range from insignificant but catchy tunes to such standards as "Everything Happens to Me" and "What a Difference a Day Makes."

These last numbers were recorded in 1960 and rank among Quebec's finest moments in the studio, certainly as a balladeer. Not only is he in top form, unfurling one seductive chorus after another, but his band mates -- organist Sir Charles Thompson, bassist Milt Hinton and drummer J.C. Heard -- provide him with a truly inspired setting.

Teddy Bunn "The Pete Johnson/Earl Hines/Teddy Bunn Blue Note Sessions" (Mosaic MR1-119) turns back the clock to 1939 and '40, a period when Blue Note was recording traditional jazz and boogie-woogie musicians.

Although the legacy of Johnson and Hines is readily available elsewhere, these rare piano sessions are certainly worth investigating, if only to hear Hines' remarkable improvisations.

Guitarist Bunn, of course, wasn't in Hines' league -- who is? But the five solo performances by Bunn that appear on this single album collection are revelations just the same.

Remarkably sophisticated for their time, the best of them, including a delightful version of "King Porter Stomp," are full of intricate single-note runs, rhythmic drive and blues expression.

Chances are slim that Mosaic will release this material on CD in the near future, since the company leases the material from larger firms not likely to pass up the potentially lucrative CD market themselves. Mosaic recordings are available through mail order only. The address: 197 Strawberry Hill Ave., Stamford, Conn. 06902.