Test-Driven Development describes in detail this technique from Extreme Programming. In addition, the author spends some time teaching the reader a useful set of mental tools for writing better code. TDD is a very fast read, but it is full of useful information. If I wanted my developers to only read one small book about software development, this would be it.
Note: The back of the book lists it as “Software Engineering/Testing”. This is incorrect. Test-Driven Development is not about testing, it is about programming.