CVS comes with a complete set of documentation in the form of info pages, known as "The Cederqvist". If it is properly installed, you can browse it by typing in
info:/cvsinto the locationbar of Konqueror, or you can just choose → in Cervisia. An on-line PDF version of the Cederqvist is available on the web.
As this book is maintained together with CVS, it is normally the most up-to-date reference; nevertheless, considering other documentation for learning to use CVS is recommended, in particular the following.
Karl Fogel has written the excellent book Open Source Development with CVS. About half of this book is about the development process of Open Source software. The other half is a technical documentation of CVS. Thankfully, the technical part of the book has been made freely redistributable under the GPL, so that you can download a HTML version of it. A list of errata is available on the web page mentioned above.
CVS issues are discussed on a dedicated mailing list.
There is USENET group
comp.software.config-mgmtdedicated to configuration management in general. CVS is only marginally a topic in this group, but nevertheless it may be interesting for discussing merits of various revision control systems compared to CVS.