Disclaimer: the support, if any, available from the various sites mentioned below may vary; see Getting Help if you need help.
The book "A Practical Guide to Linux" includes 16+ pages on CVS and material about RCS, GCC, Emacs, and other programs.
The "CVS Reference Card" offers a complete list of CVS commands and options for the command line CVS.
"Version Management with CVS", is the main manual for CVS. It provides details on all CVS features, and also contains introductory material.
There are a number of CVS tutorials or training manuals on the net. They are designed for introducing new users to CVS or providing to each member of a team which is using CVS. We would suggest the "Introduction to CVS", by Jim Blandy. You may prefer one of the others (although we should warn that some are out of date).
The "CVS FAQ" to a fair extent duplicates material which is available elsewhere . Nevertheless, it has a certain following and may explain some topics which are not covered well elsewhere. The biggest caveat is that many parts of it are out of date.
Also see our books page. At least one published book contains significant CVS content.