The butler did it.
Like not enough biographies of musicians, this one involves Satie's life and music both. Without getting very technical about the music (which in Satie's case isn't very technical anyway), Mary E. Davis explains how Satie composed, how he used source material like cabaret or pop songs, and to a certain extent what his artistic goals were. She also discusses his life as an artist among the beaux-arts, décadents, avant-garde, and Dada that he played with.
The bio never gets deeply into any particulars of any of this, which is rather like Satie, ain't it?