The Grim Phoenix is an excellent piece of early scholarly criticism on Thomas Pynchon. Since it cannot rely on the vast body of criticism published since the late 1970s, it goes deep into the early novels and short stories. It is a good choice if you want to soak in interesting ideas without being bogged in postmodernist theory and countless reference works.
Just stumbled onto this at Moe's Bookstore in Berkeley, CA. Very informative and thoughtful, yet clear in the discussions of this sometimes-difficult author's work (up to 1978, at least, when this study appeared).