Bass player extraordinaire Charles Mingus, who died in 1979, is one of the essential composers in the history of jazz, and Beneath the Underdog, his celebrated, wild, funny, demonic, anguished, shocking, and profoundly moving memoir, is the greatest autobiography ever written by a jazz musician.    It tells of his God-haunted childhood in Watts during the 1920s and 1930s; his outcast adolescent years; his apprenticeship, not only with jazzmen but also with pimps, hookers, junkies, and hoodlums; and his golden years in New York City with such legendary figures as Duke Ellington, Lionel Hampton, Miles Davis, Charlie Parker, and Dizzy Gillespie.  Here is Mingus in his own words, from shabby roadhouses to fabulous estates, from the psychiatric wards of Bellevue to worlds of mysticism and solitude, but for all his travels never straying too far, always returning to music.   "This book is the purest of dynamite.  Like the autobiographies of Jelly Roll Morton, Louis Armstrong and Billie Holiday and like A. B. Spellman's Four Lives in the Bebop Business, it says more about the American psyche in general and black survival in particular than the sociologists and psychologists ever can in their stiff, soulless vocabularies.... Somber, comic, disturbing, boastful, confessional, sentimental, contradictory, poetic, irascible, impish...lyrical, nasty, angelic, reflective...expressionistic, picaresque, jive...this is a powerful book."-- Rolling Stone