From the back of the book:
Aliens! These oftimes strange and unique beings fill the Star Wars saga with variety and excitement. Fighting alongside aliens with the Rebel Alliance? Engaged with them in something the Empire considers a less-than-legal activity? Need to know more about these aliens?
Alien Races provide the information you'll need on homeworlds, cultures, lifestyles and the behavior of over 30 alien species. Whether recognizable as bipeds, or insectoids with compound eyes, they make up a huge portion of the Galactic Empire.