A music of wiggy insularity with stars named Dizzy and Bird, and onomatopoeic subgenres like bebop jazz is one form of American entertainment that, to the average eyes and ears, has always needed a translator. And the man who broke the code for millions of readers and listeners since the 1930s was Leonard Feather, who died Sept. 22 at age 80 of complications from pneumonia. An expatriate Briton, Feather developed an early and profound understanding of an essentially African-American art form, and he expressed it with casual grace in 10 books published over the past five decades. The cornerstone of his career is surely the standard reference source The Encyclopedia of Jazz (1955), which he was updating at the time of his death. Feather's muse was as adventurous as the music he treasured. In addition to his books, magazine articles, and countless pieces for the Los Angeles Times since the late 1950s, he wrote the music and/or lyrics for more than 200 songs (including ''How Blue Can You Get'' and ''Evil Gal Blues'') that were recorded by artists ranging from Duke Ellington to Duane Allman. He tripled as a producer, too, and was the first to record pianist George Shearing and singers Sarah Vaughan and Dinah Washington. ''Leonard had exquisite taste,'' recalls the saxophonist-composer Benny Carter, who serenaded Feather at his bedside every day after his longtime friend slipped into a coma. ''If he liked it, it was good. And when he said it, you understood.''