The Encyclopedia of Science Fiction is more than just a reference book - it's clearly a labour of love from the authors and many contributors. Sections are diverse, covering hard sci-fi authors, sci-fi publications and even a large section on 'Star Trek'. There are popular movies from the world of speculative fiction, well known tv shows and even areas devoted to the world of comic books and comic book heroes (notably DC and Marvel, the big two.) It's easy to use and I've had no problems using the cross reference system to look up information for my 'Writing Sci-Fi/Fantasy' class. The only areas in which it falls down perhaps, are in comparison to it's sister title, Encyclopedia of Fantasy
and the fact that it's now a little outdated. The Fantasy version is easier on the eye and can honestly (in my opinion anyway) be read for entertainments sake and if you're looking for anything written/shown on the big screen since the early nineties, you'd better look elsewhere, though I hear there is another, up to date version in the pipeline. Despite this, the greats are all here - Asimoc, Dick, Bradbury, etc.
At the very least you'll get a fantastic introduction to the world of speculative fiction and a decent reference material for anything deemed 'Sci Fi'. A must for any hard core fan or a student studying the topic. Buy the fantasy version too!