CVS comes with a complete set of documentation in the form of info pages, known as "The Cederqvist". If it is properly installed, you get browse it by typing in info:/cvs into the locationbar of kdehelp, khelpcenter resp. Alternative, you can just choose Help->CVS Info in Cervisia. An online HTML 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 I recommend considering other documentation for learning to use CVS, in partical 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 webpage mentioned above.
CVS issues are discussed on a dedicated mailing list.
There is USENET group comp.software.config-mgmt dedicated 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.
Last but not least, there is a (low traffic) Cervisia mailing list.