Algernon's Pad is Daniel Keyes's web site. There's a biography, bibliography, and FAQ section. In the bibliography, each book is accompanied by a description and reviews.
Authors and Artists for Young Adults. Vol. 23. Gale Research, 1998
Daniel Keyes: 40 Years of Algernon, excerpted from Locus Magazine, June, 1997.
Locus Magazine has an excerpt from a June 1997 interview they conducted with Keyes.
Wired for Books has an audio interview with Daniel Keyes, which was conducted by WOUB, Oxford.