The BX Advanced Bestiary, Volume 1, takes the monsters in the OSE Basic book alphabetically and expands on them, offering variants, special abilities that can be added, and flavor and background to make them more interesting.
Purchase includes the book, an 8-1/2 x 11 book with sewn binding, ribbon bookmark, and black and white interior. There are over 70 new monsters included in this book, each illustrated.
It includes rules for monster and animal training, several new race-as-class options (advanced cave locust, changeling, cyclopskin, dragonborn, and more), as well tables showing the monsters organized by HD, terrain, and type.