Although this manual is slightly out of date (published in 2007), the market hasn't changed much since then so the indicative guitar prices are still valid.
One of the real plusses I found with this book were the references about how to determine whether a guitar is in "excellent" or average (for example), conditionfor it's age. There are both visual and written pointers. Essential when you're thinking of spending fairly big bucks for something that's second hand - especially if you're buying online. This will guide you with exactly what questions you need answering and what you'll need to see before you commit.