> How come the orchestras behind Billie Holiday were so amazingly good?...and she sang like one more instrument among the others ...in some of the purest and best jazz I've ever heard...
Holiday was born in Manhattan, and her father was in Fletcher Henderson's band, pretty much the best black orchestra working the white society circuit. At the time, social music was available in Manhattan such that Holiday could be singing in small clubs, and have folks like Benny Goodman in the audience, who I believe was the first to record her. That put her in touch with John Hammond the promoter and Café Society, which was really a center of the whole move to treat jazz as an art form.
Holiday began her career with the Count Basie orchestra. In a sense, she started at the top. Lester Young, who was also in the band at the time, was a champion of her music.
You should try to find her early "swing song" recordings with a subset of the Basie band: Young, Freddie Green on guitar, Walter Page, Jo Jones. The swing song treats the vocal as an equal instrument: after the opening theme, each instrument (including voice) takes their turn with the song and then it's done. Very simple, but it offers wide room for interpretation, individual expression and trading. Holiday said that her vocal inspiration came from Louis Armstrong who also had a very limited range.
HTH, Charles