I started unit testing a handful of years ago, and I've read quite a few on it since my initial book.
However, my initial was "Test Driven" by Lasse.
For me, the author made it simple to understand.
Perhaps you could pull some info from it for your teaching.
And btw, I've taught TDD as well.
I have found that ensuring the audience understands how to use unit tests before going into TDD to be quite handy.
Good luck! :-)
