by Gary K. Wolfe, In Other Worlds: SF and the Human Imagination by Margaret Atwood, Trillion Year Spree: The History of Science Fiction by Brian Aldiss,The Language of the Night by Ursula K. Le Guin, as well as numerous articles and essays, including Peter Nichols and John Clute's excellent online Encyclopedia of Science Fiction. The meta of science fiction as a literary and cultural movement is just as interesting as the stories themselves. Bring on Thomas Disch's 1998 The Dreams Our Stuff Is Made Of: How Science Fiction Conquered the World.
Of all the non-fiction sf I have read, The Dreams Our Stuff Is Made Of is perhaps the most erudite (despite the title ending on a preposition, natch). Disch brings to the table his experience as a writer of many forms of fiction and poetry (not just sf), his work as a published critic, columnist, and essayist, not to mention broad reading experience outside the genre. Like Aldiss and Atwood, Disch is better positioned than the average genre writer to form an opinion about the context and evolution of science fiction in the world at large.