The Encyclopedia of Science Fiction
Newly expanded and now online, a wonderfully opinionated and scholarly reference. Also some terrific writing: here’s part of the entry for Kathryn Cramer:
she has been involved in various capacities with the New York Review of Science Fiction since it began in 1988, where she has published some spiky, erudite criticism. She has become deeply involved in arguing the aesthetic case for – and writing – fiction designed for Hypertext, including "In Small & Large Pieces" (1994 The Eastgate Quarterly Review of Hypertext #3); given rapid advances in the necessary technologies, her advocacy – which seemed premature in the 1990s – may yet seem to have been predictive.
The entry on hypertext, which I understand to be the work of Neal Tringham, is extraordinarily good – perhaps the best short introduction to hyperfiction yet published.