John Clute, David Langford and Graham Sleight (eds.), The Encyclopedia of Science Fiction (third online edition due soon - see http://www.sf-encyclopedia.com/; the second edition, ed. John Clute and Peter Nicholls, London: Orbit, 1993, corrected paperback 1999, is also worth consulting)
Adam Roberts, The History of Science Fiction (Basingstoke: Palgrave Macmillan, 2005)
David Seed (ed), A Companion to Science Fiction (Oxford: Wiley-Blackwell, 2005)
The following works are not perhaps so essential - some of them are primarily about fantasy, but have useful insights for sf, others are on subsidiary areas of sf. But they do come highly recommended (and not just by me):
Farah Mendlesohn, Rhetorics of Fantasy (Middletown, CT: Wesleyan University Press, 2007)
Farah Mendlesohn, The Inter-Galactic Playground: A Critical Study of Children's and Teens' Science Fiction (Jefferson, NC: McFarland, 2009)
I'm open to comments here. Are there any obvious texts I've missed? I won't invite you to argue that there are works I've included that shouldn't be on this list, because I think they all should be.