"A Complete Source"
Bachman explores language testing from a theoretical point of view in this book. I found the coverage of measurement, testing purposes, reliability, and validity to be very useful. Perhaps the one warning I would give is that this book is a little statistics-heavy, and it assumes that you have at least some knowledge of statistics. If you don't have any knowledge of statistics, then the sections on reliability and validity may take a little work to get though. In that case, I recommend Brown's text on testing. It's a simpler read. However, this volume by Bachman is more detailed and Bachman is rightly considered to be an authority on testing.