SourceGear is a company that I’ve never heard (until now) that works on version control software. Eric Sink is the founder of SourceGear, and he has recently written and published the book Version Control by Example. I came to hear about it perhaps as many others who have seen it already would have — he is giving this book away for free (shipping costs covered), and as his FAQ page says, there are no string attached.
The book covers the history of Version Control Systems (VCS), the terminology and common feature sets, examples of each of those features (in Apache Subversion, Mercurial, Git, and SourceGear’s Veracity), a discussion of how to use a VCS effectively in practice, and details about how VCS’s work internally. Through following VCS’s throughout history, we are introduced to additional VCS features in a context that easily explains their motivations.