Of course, CVS is becoming somewhat irrelevant these days (except for the public project hosting services, where it prevails), but this book is of moderate size (368 pages), packed with useful information (if you still have to use CVS and wish to upgrade your knowledge somewhat), written by world-famous developers (Karl Fogel of CVS and Subversion fame, Moshe Bar of openMosix, Xen and KVM fames), and is available freely on the Internet (under the GNU GPL).
Book covers CVS basics, repository administration, and some advanced (for CVS world) features. It contains extensive CVS reference, has many tips and troubleshooting advices, and briefly describes various third-party tools which make the CVS more useful.
Read it online: Karl Fogel, Moshe Bar “Open Source Development with CVS”; download in PDF form; see the book’s official site.
Buy at Amazon: Open Source Development with CVS.
More books on version control and SCM: Version Control Blog Library.