The Advanced Player's Guide presented alternate racial traits for all core player races, as well as new benefits for taking a level in a PC's favored class.
Monstrous Races takes the opportunity to stand on the shoulders of giants by expanding this material to cover the non-traditional player races. Inside you will find at least eight alternate racial traits each for aasimar, drow, duergar, goblins, hobgoblins, kobolds, tengu and tieflings. Each race also has a section on substitutes for favored class benefits other than an extra hit point or extra skill point.
The Advanced Player's Guide allows players to never play the same character twice and allows gamemasters to never use the same opponent twice. Monstrous Races makes sure both players and gamemasters have even more options.