The book Open Source Development with CVS is available in electronic form from the book's web site.
The reference manual for CVSNT™ is available here.
Development of TortoiseCVS has been made a lot easier thanks to a number of other projects:
wxWidgets is a GUI framework, which is used for all dialogs in TortoiseCVS.
CVSNT is the client that TortoiseCVS uses to communicate with the CVS server. Commercial support for CVSNT is offered by March Hare.
Inno Setup is used to build the installer for TortoiseCVS.
Cygwin provides a number of UNIX tools that make building TortoiseCVS a lot easier.
eDE is a DocBook environment used for building the TortoiseCVS documentation.
CppUnit is a unit test framework which is used for testing some parts of TortoiseCVS.