The AACM is a truly wondrous outfit, arguably unparalleled in the contemporary United States. It is a collective of avant-garde musicians playing "great black music", that has sustained itself over the course of several decades ~ truly a monument to cultural, artistic and organizational creativity. Among the reasons I was so excited to go off to graduate school in Chicago in the late 1970s was the prospect of hearing music by members of the association. Happily, I managed to take in quite a number of performances in the many years I lived there. I have posted on some of the AACM's guiding lights here and here.
Update ~ (11 May 08): Here are a couple of pieces from The New York Times prompted by the release of Lewis's book  .