Quite simply, anyone interested in the history of avant-garde film and its major filmmakers needs to pick "Visionary Film" up. It's one of the first, and still one of the finest, major texts to be published on the subject. Sitney has dedicated the majority of his life to the study of American avant-garde filmmaking, and it shows; he brings to his work both a breadth of knowledge and a depth of insight that are pretty much unparalleled.
It's quite a dense text, and some of the films discussed in the book can be very difficult to see, due to any number of distribution issues and the fact that many of the filmmakers discussed here have only exhibited their works on film. Still, you shouldn't let either of these things dissuade you from reading it; it's incredibly rewarding regardless.
One Note: The subtitle to the third edition here is somewhat misleading, since the book, originally published in 1974 (with a 2nd Ed. in 1979), focuses primarily on filmmakers and films from the early 1940s to the late 1970s. The final chapter of the third edition essentially acts as a brief summation of trends in the avant-garde from the end of the 1970s up to the end of the 20th century. It's certainly informative, but cursory compared to the rest of the book. Those disappointed by this, or anyone looking for a more detailed study of major avant-garde filmmakers and works during this time period should also take a look at Sitney's latest book, "Eyes Upside Down," which I am currently working my way through; so far, it too is exceptional.
