Speculations on Speculation: Theories of Science Fiction - an excellent theoretical overview of what SF is, how it has developed, and where it's going. Some of the essays are somewhat elderly now, as it's a collection of essays from previous anthologies, but it's very useful to find a book that actually links SF with other literary traditions and explains the similarities and dissimilarities between them. I was very interested in Darko Suvin's essay on the device of cognitive estrangement (basically the same as a Brechtian Verfremdungseffekt). I'm not sure if I think it is a pre-requisite for SF, but it probably is. Personally I think the question "What if...?" is usually the starting point for SF. It should also include some kind of science I suppose, though so-called soft sciences are just as valid a basis for SF as the hard sciences, in my opinion.
Also reading Catherine de Medici: A Biography by Leonie Frieda. Fascinating stuff and reasonably well-written; having seen the film La Reine Margot, it's a period I'm quite interested in. Also there's usually more than meets the eye to people who have a really bad reputation - it's often a distorted picture of what they were actually like (take Machiavelli for example). I don't normally read biographies, but I like history stuff. The only drawback with Catherine de Medici: A Biography is the almost Byzantine complexity of the politics of the time and the interrelationships of the protagonists, plus the number of times she uses the word 'superb'. I wish authors would not do this (Melvyn Bragg had a nasty attack of it in The Adventure of English, where he kept repeating the image of the English language drilling deep holes into the bedrock of something or other; though apart from that it was an interesting book).