I go in cycles.
Previously: I read almost nothing but science fiction. From the classics (Asimov, Clarke, et al) to the modern masters (Brin, Benford, et al) to the occasional bit of pulp (Zahn, Foster, et al).
Then I had a period where I read almost nothing but science nonfiction. The God Particle, Kaku’s Hyperspace, Gleich’s Genius (and several of Feynman’s lay-aimed texts), The Selfish Gene, The Beak of the Finch, Sagan’s Comet, and so on.
Then I had a period where I read almost nothing but cinema-oriented books. Film theory and history, biographies, memoirs mixed with theory (Lumet’s On Directing Film is quite good), the Faber & Faber “[filmmaker] on [filmmaker]” series, annotated screenplays, and so on.
For a brief period, I read a lot of graphic novels or comics in collected volumes (Bone, League of Extraordinary Gentlemen, Preacher, etc.).
Right now I’m in a general nonfiction phase. Recent titles include The Map that Changed the World, A Primate’s Memoir, and a rereading of Mr. Wilson’s Cabinet of Wonder and Godel, Escher, Bach.
Basically, this is because I’m a really, really fast reader. I’ll get into an area and read a whole lot of the best stuff. Then I “run out.” Not really, of course, considering how many tens of thousands of books are published every year, but I get through all the stuff that interests me at the time. Then I move on to another area. When that’s exhausted, I may come back to a previous area of interest, because good stuff will have built up in my absence.
I tend not to read two major areas: General fiction (whether literary, like, say, Kazuo Ishiguro (sp?), or mainstream, like Grisham), mostly because there’s so damn much to choose from I rarely get something I like; and business/self-help. (I lump those together for idiosyncratic personal reasons, which if I were to explain them would clarify why I don’t like the genre.)