Equipment is the most useful and a favorite reference book. Inside are hundreds of items, even non-magical. Since it’s very thorough, for my game, it is the de facto source for gear, overruling the Core Rulebook and (I think) GMG. The bulk of Equipment organizes ‘wondrous items’ by which ‘body slot’ they occupy (feet, torso, waist, etc.). Thus, it is a must have rulebook. It’s also available in the smaller, cheaper pocket edition.