Ebert loves this book, recommending it as the best all-round introduction for the movie-goer who wants to know more about the way movies are made. Lumet doesn't gossip; he just sits down and explains how things work: screenwriting, directing, lighting, camerawork, even a little bit of focus group marketing.
Some of this is perhaps a bit too elementary, and some its perhaps too compressed. I'd have liked, for example, to understand where Lumet's approach to directing differs from, say, Hitchcock, or Coppola.
I read this just after reading Mamet's On Directing Film. Perhaps that was unfortunate. My first impression was that Making Movies was not particularly well written, but on review I think it's just a matter of lacking Mamet's luster.