Rick Hower maintains a web page which includes a nice list of web
testing tools, and some guidelines for testing web sites.
<http://www.softwareqatest.com/qatweb1.html>  Please contact him at
rickhower@earthlink.net if you can offer additions or corrections to the
list.

One book that can be of help, "Testing Applications
on the Web" by Hung Q. Nguyen and "The Web Testing Handbook" 
by Steven Splaine and Stefan P. Jaskiel.

One recent addition is Quality Web Systems by Elfriede A. Dustin/Douglas McDiarmid.
<http://www.StickyMinds.com/books.asp>
It also contains a Tool Evaluation appendix, comparing the
major tools (Mercury, Rational, Empirix, Compuware, Segue).
This matrix consists of approx. 30 pages of detailed
evaluation criteria.