My rule of thumb is to never buy a guidebook with a copyright date older than a year ago. The information in the guidebook is already 6 months to a year old, as there is the time needed to print and distribute. I'm spending both time and money to get to a destination; the $20 spent on a guidebook is money well spent to maximize my pleasure once there.

Guidebook publishers can make it very difficult to find the copyright date, burying it within the book. If I can't find the copyright date, I don't buy the book.

My exception is the Michelin Green Guides, which are purely for sightseeing. Sights don't change as quickly as food and lodging.