In addition to this book, there are a number of other helpful CVS resources that you should know about. CVS has an active user community, and additional resources may become available over time.
The CVS home page is at http://www.cvshome.org/. This site provides a copy of the info cvs manual by Per Cederqvist et al, a CVS FAQ, and links to most of the available third-party tools for CVS.
On Unix and Linux systems, the man cvs and info cvs commands provide documentation for the server and the command-line client.
The CVS Pocket Reference by Gregor N. Purdy is a useful take-anywhere book published by O'Reilly & Associates, Inc.
Applying RCS and SCCS by Don Bolinger and Tan Bronson explains RCS, the tool CVS calls to manage individual files within its database. It is also published by O'Reilly.
Open Source Development with CVS by Moshe Bar and Karl Franz Fogel contains a guide to CVS and a guide to open source project management. The print version is published by Paraglyph Press, and sections of the book are available online at http://cvsbook.red-bean.com/.
Version Management with CVS by Per Cederqvist et al is published online at http://www.cvshome.org/docs/manual/ and is also available as info cvs. The print edition is published by Network Theory Ltd.
Linux Server Hacks by Rob Flickenger is published by O'Reilly & Associates, Inc. and contains 11 hacks for using CVS as a system administrator.
CVS is an open source project. While there's no one company backing CVS and providing support for it, you can get a wealth of support from your fellow CVS users. The official CVS mailing lists are at http://www.cvshome.org/communication.html. The most important are email@example.com for discussion and support and firstname.lastname@example.org for bug reports.