Paizo’s second of six Bestiary reference books for GMs. The first one was pretty important to have because it contained the most common monsters in Fantasy, namely other humanoids (goblins, orcs, gnolls), dragons (mandatory), giants (again, mandatory) and demons. B2 is closer to be being part two since there are about 20 entries which usually resided in WOTC’s first Monster Manual; other lycanthropes (werebear, -tiger and -boar), beasts such as the hippogriff, magical allies the blink dog and hippocampus (a giant seahorse, not a part of the brain). Paizo’s creations from their earlier Adventure Paths are present too: Leukodaemon and the Rune Giant.
Is it worth getting, a resounding yes, especially when B2 now comes in the $20 pocket format. It also helps to have on hand due to Paizo modules saving space (rightfully so) by just referencing the source for a typical version of a monster. More room to write in important details: traps, tactics, motives, room descriptions, etc.