There are a couple of good books for learning Unit Testing (in particular Test-Driven Development) for Embedded Development (in C!). It'd be nice if there were more, but these should get you started. If you're the adventurous type, you could also read some of the fine books on Agile and TDD for other languages. Such books were invaluable in developing these tools in the first place.

This is a mini-book which focuses specifically on C using Unity and CMock. It was written by one of the tools' primary authors and covers all the basic concepts. It comes as a square softcover book or as a $5 DRM-free ebook. It's not the best book for learning TDD flow, but is a good reference for these tools in particular. (rumor has it that a new version is nearly complete)

This book was written by James Grenning, one of the original signers of the Agile Manifesto. It's a good introduction to learning TDD and how to apply it to C code. James's book uses a combination of Unity and CppUTest. It introduces mocking (and other fakes) but doesn't get in depth. Also, note that all Unity code uses the fixtures addon instead of the standard Unity interface.