Working Effectively with Legacy Code by Michael Feathers
"...legacy code is simply code without tests... Code without tests is bad code."
From those statements, it doesn't take much to figure out what this book is about - how to write unit tests for code without tests. Of course, if you've ever tried to do that, you know that it's easier said than done. Many/most programmers who inherit a big ball of mud of code without tests (legacy code) just punt; the existing code has no tests, I can't see how to get any of it under test, so I'll just hack and pray - just like the original author(s) did.
That's where this book comes in. It's a primarily large collection of recipes about how to write unit tests for legacy code. That said, the focus is not really on how to write the tests but rather how to get chunks of the legacy code into a test harness so that you can write unit tests to characterize the existing functionality before adding or modifying functionality. It also contains techniques to add new functionality in such a way that you can test it immediately and possibly execute the "clean up" that the original author(s) promised would happen as soon as that next deadline was reached - all those years and deadlines ago.
The bulk of the book (Part 2) is organized as a series of complaints or excuses and how to deal with them. These include such topics as "My application has no structure," "Dependencies on libraries are killing me," and "This class is too big, and I don't want it to get any bigger." In each chapter, the author provides examples (in multiple languages - Java, C++, C, etc.) of these problems and specific techniques that can be used to address them. The last chapter (Part 3) is an encyclopedia of the techniques for easy reference.
If you're lucky enough to do only green-field development, you might think this book would be useless. However, one interpretation of this books is that it is a list of sins to avoid while your playing in the green field. And, many of the techniques can be interpreted as best practices for how to write you code to ensure it's testable. (Of course, you're following test driven development and achieving near 100% coverage, so that would never be a problem with your new code, would it? :-)
My one disappointment with this book was that I was hoping it would provide ideas about how to create higher-level (e.g., functional) tests. Of course, high-level tests are no substitute for unit tests. It's just that I was tasked with creating "some tests" quickly for an entire application, and unit tests are not practical in this particular case, which is my problem, not the author's.
This book is an excellent resource and cookbook for how to add unit tests to an existing code base that lacks tests, and it also provides design and implementation templates to ensure that new code is testable as it's created.