Black Holes & Time Warps: Einstein's Outrageous Legacy, by Kip S. Thorne, W. W. Norton & Company, 1994.
Gravity's Fatal Attraction: Black Holes in the Universe, by Mitchell Begelman and Martin Rees, Scientific American Library, 1998
Other books with good information on black holes:
The Hundred Greatest Stars, by James B. Kaler, Copernicus Books, 2002
The Physics of Star Trek, by Lawrence M. Krauss, BasicBooks, 1995
Time Travel in Einstein's Universe, by J. Richard Gott, Houghton Mifflin, 2002
Other websites with good information on black holes: