• Maximum RPM (online version)
  Currently the best reference (except for the source itself, that is).
  Lacks some updating.
• The RPM HOWTO from the LDP
  There is a resonable chance that you have a local copy of this document on your system. It serves better as an introduction. Lacks some updating
• Mandrake RPM HOWTO
  More up-to-date, but has more mandrake-spesific details.

the rpmfind database
Useful for finding who supplies a dependency, and for comparing packages of different distros.