While you've already accepted an answer to your question I'd like to recommend a few other books not yet mentioned:
- Working Effectively with Legacy Code - Michael Feathers - As far as I know this is the only book to adequately tackle the topic of turning existing code that wasn't designed for testability into testable code. Written as more of a reference manual, its broken down into three sections: An overview of the tools and techniques, A series of topical guides to common road blocks in legacy code, A set of specific dependency breaking techniques referenced throughout the rest of the book.
- Agile Principles, Patterns, and Practices - Robert C. Martin - Examples in java, there is a sequel with examples in C#. Both are easy to adapt to C++
- Clean Code:A Handbook of Agile Software Craftsmanship - Robert C. Martin - Martin describes this as a prequel to his APPP books and I would agree. This book makes a case for professionalism and self-discipline, two essential qualities in any serious software developer.
The two books by Robert (Uncle Bob) Martin cover much more material than just Unit testing but they drive home just how beneficial unit testing can be to code quality and productivity. I find myself referring to these three books on a regular basis.
