I was just recently sent a review of my book An Introduction to Statistical Problem Solving in Geography. If you are thinking about a good introductory text on quantitative geography, this review will give you a good idea of what the book is about.
Also, for a limited time, you can get my online version of the course for $30 here. That’s 12 hours of lectures, explanations, and hands-on demonstrations – these are similar to the lectures I give in my University course (although somewhat abbreviated). So, if you want apply quantitative geographic theory in your GIS work, this is a great way to learn.