Wearing Two Hats in Harlem : BLUE NOTES UNDER A GREEN FELT HAT <i> by David Ritz (Donald I. Fine: $18.95; 244 pp.) </i> : BLUE FRUIT <i> by Adam Lively (Atlantic Monthly Press: $16.95; 136 pp.) </i>

Although they have in common a partial involvement with the world of jazz, these novels are about as similar as a comic book and The Atlantic Monthly.

The curious title of “Blue Notes Under a Green Felt Hat” derives from the no less curious contrivance by which the author brings together the disparate worlds of his characters. The main participants are Danny Klein, a philandering Damon Runyon type who is torn between his love of hats and his passion for jazz; Myron Klein, his philandering father, a famous hatmaker who wants his boy to go into the business; the philandering Mama Klein, who is involved with the family doctor (“He’s a wonderful man, and his children’s complexions have improved a hundred per cent”) and Danny’s possibly philandering sister, who has fallen for a jazz critic, Winthrop Carrington. When Winthrop opens his pompous mouth he seems to have swallowed an encyclopedia of jazz cliches.

On the other side of the fence is Clifford Summer, a black singer and pianist who loses two fingers during a 52nd Street shoot-out. Danny befriends Clifford and will see to it that he makes more money playing with eight fingers than he ever did with 10.

How will these two protagonists’ lives converge? How can Danny stay in the family business while retaining his interest in jazz? Why, simply by wearing two hats--he opens a shop in Harlem that sells hats and records, and gets it under way by inviting Billy Eckstine to autograph both during the grand opening.

Along the way, Danny finds himself in the Deep South visiting Clifford’s mother, a wise-old-woman type who spouts such memorable advice as: “It’s not knowledge that serves the heart, Danny, it’s wisdom.” With this in mind, Danny will be able to unload plenty of fedoras.

His romantic problems are an ongoing thread in the tangled narrative: Shall he stay with the stripper from New Jersey, or the intellectual who takes him to readings at the New School? And how will he adjust to having a jazz-history-spouting brother-in-law?

Ritz builds to a not-unpredictable trick ending, one at which he has dropped a few detectable hints along the way. By peppering his story with a wealth of informed jazz references, and by equating profanity with authenticity in the dialogue, Ritz attempts to create a genuine Zeitgeist. He even wants to convince us that a song by Clifford (“Up in Harlem where swing’s the thing, be-bop blues is the newest thing . . . Be-bop blues really rocks and rolls”) could have become a major hit in 1949, when much of the action takes place.

The main problem, trivial ditties aside, is that David Ritz is to literary grace what Robin Leach of TV infamy is to vocal style.

Ironically, though Adam Lively’s “Blue Fruit” makes no attempt to simulate reality, it captures the reader immediately through its fantasy-based premise and sensitive narration. This is the first novel by a London-based 26-year-old author, educated at Cambridge and Yale.

The story is a latter-day “Gulliver’s Travels.” In 1787, John Field, a young ship’s surgeon in the South Seas, finds himself lost and is put ashore in what turns out to be 20th-Century Harlem.

This chance-taking premise works out delightfully. Meeting a black man in this strange new world, he asks him: “Are you a slave?” Totally lost in his new environment, he is taken in by a black family and befriended by a young saxophone player. Somehow the two men forge a common bond, not only socially but in music: A classically trained violinist, Field transcends two centuries of musical evolution and is able to express himself creatively at a jam session with the saxophonist’s group. In due course, however, he is confronted with the realities of race relations and the tensions of the sophisticated world in which he has inexplicably landed. (The exact time frame is never clarified, though certain references such as a mention of the Clef Club would seem to indicate a period not long after World War I.)

In this haunting, almost hallucinatory tale, we are not told enough about Field’s reactions to some of the technical realities of our century: We never learn, for example, how he feels on first seeing an automobile. But the areas Adam Lively does cover are subtly pinpointed; the sequence in which he tries pot for the first time is richly developed, in contrast to a similar episode in the Ritz story.

“Blue Fruit,” with its mix of fantasy, history and cultural contrasts, is a strikingly original work that presages for Adam Lively a future worth watching.