Differential GPS explained: An exposé of the surprisingly simple principles behind today's most advanced positioning technology

The book came on time and only the outside cover is scratched up. Considering I'm only using this as a reference for my master's thesis work it's perfect! And who am I to complain when it was only $8!

This is a good book for explaining GPS to people who know nothing about it. It is old, but most of the basic facts remain true.