This is the first book I ever read on the subject, and it still remains one of my all-time favorites. Despite the fact that several portions of the book are strictly D&D rules (such as the classes, specifically Elves, Dwarves, and Halflings), most of the remaining materials can be easily adapted to any version of AD&D, and in most cases the DM will benefit from doing so. This book covers in near-completion such viable topics as the specifics of combat, such as non-lethal weapons combat, special maneuvers (set lance versus charge), and weapons mastery and training, higher levels and the trappings that come with the rank (castles, followers, assistants), and quite extensive listings of very interesting monsters, most of which were never seen in any other place again (among which there are several truly alien extraplanars and a few utility creatures).
Best of all, this book presents all of these necessary rules in an easy and comprehensive form that does not destroy the original rules in favor of new ones. If you can find this handbook for a good price, I say buy it.