Getting a Game Master's Guide together for a role-playing game is pretty hard work. Everyone has a different opinion of what it should include. Thanks to the fact that the Pathfinder RPG already has all the necessary rules information in its Core Rulebook and Bestiary, Paizo manages to dodge this difficulty a bit with their GameMastery Guide, focusing on advice and tools that both new and experienced GMs can use. While there's a little rules information, the book is mostly chocked full of advice and tools ranging from random treasure tables to dozens of ready-to-use NPC stat blocks.
In terms of usefulness, the book is definitely going to be used more by new or inexperienced GMs. The bulk of the guide is advice ranging from getting a playing group together to world-building and designing adventures. Mind you, it's all good advice - the writers remain well-grounded in what is useful to gamers now, even bringing in technologies such as text messaging and wikis that can be useful tools for a GM. However, if you're an experienced GM, most of the advice amounts to a retelling of what you already know. The tables and stat blocks, however, are extremely useful in games, allowing any GM to speed things up by having a ready-made stat block of the city guard the PCs just enraged or a random table to roll on when the PCs discover a surprise cache of treasure. Whether this information justifies the price tag, though, is a matter of opinion. New GMs should definitely pick up this guide, as it is a great toolbox to learn how to run a game. More experienced GMs might find the book less useful outside of a few key sections. Fortunately, for the latter, there's always the option to just purchase the PDF for about $10 - an excellent compromise that allows everyone to use the wealth of material within on their own terms.
Too often in fantasy role-playing games, be they Dungeons & Dragons or another system, the most popular monsters end up being little more than clones or parodies of one another. Orcs end up being slightly tougher versions of goblins, and ogres are just bigger orcs. Classic Creatures Revisited seeks to put this problem to rest, and does so admirably.
Technically, this book uses the 3.5 edition of the Dungeons & Dragons rules. However, outside of a few sidebars, the book is almost entirely role-playing information and adventure ideas, meaning it can be used in any edition of D&D -- or, really, in any fantasy role-playing system. What this book offers is a chance to look at the old standards differently. Bugbears go from being big goblins to the bogeymen that hide under children's beds. Kobolds are sneaky, devious creatures that strike from the shadows using traps and stealth. Goblins are psychotic and dangerous, but somehow cute in their own gremlin-esque way. Each entry has details on a monster's culture, ecology, and how they can be fit into the campaign. While the book is intended to be used in the Pathfinder setting of Golarion, it is almost entirely generic, and can be ported over to any game.
This book offers the most valuable resources of all: the chance for Game Masters to get a fresh look on creatures and new ideas for all manner of terrific adventures. From a player's perspective, it's a wonderful read and a chance to get some insight on some classic fantasy monsters. I can't recommend it highly enough.