The first of six monster books from Paizo at the time of writing this and the perfect companion for the GM since the players need monsters to encounter and/or defeat.
Originally I had shied away from the Bestiary series because I already owned all the core monster books for 3.5e. There wasn’t much to convert from 3.5e to Pathfinder plus Bestiary lacks some beloved opponents via the “gentleman’s agreement,”: mind flayers, kuo-toans and the Gith races. I did come around due to the convenience Paizo’s books offered through the stat blocks. As I’ve griped before, I don’t agree with the changes to the Undead but it isn’t hard to convert them back to their more superior position.
Another selling point with Bestiary is that Paizo’s take on core monsters is quite different and this is good, otherwise you’re wasting money. The differences between traditional D&D v. Pathfinder is the latter’s art work. Goblins have oversized heads, trolls resemble boars in their facial features, hobgoblins aren’t as tall as humans, etc. Goblins received the biggest alteration for they’ve become Paizo’s mascot. No longer are goblins one of the easy creatures to beat up for starting players. They’re now cunning little things with a penchant for fire, fear of horses and dogs, and an explanation for their illiteracy. In a direct fight, they’re easy to beat but goblins know this and will resort to trickery against the overconfident players.
They didn’t waste any pages on the PC races which is perfectly fine with me. Core Rulebook does a good job explaining the possibilities of encountering a dwarven war party or a halfling caravan. The evil versions are there; drow and duergar have gone down a separate cultural track making them different races.
The last 30 or so pages gives pointers on building your own monsters, showing how they can advance and explanations of their special powers which may not be in enough detail via their entries. Combine it with the Core Rulebook, boom, you can make NPC-like monsters: Orc Fighter, Goblin Sorcerer, Lizardfolk Druid, etc.
Bestiary is a great for new GMs and it also comes in the pocket edition for a mere $20. It isn’t mandatory for 3.5e vets, I’ve found it just saved time.