From the back cover:
Each of the underground races in this volume has page after page of specialized weapons, lifestyle and religious notes, training packages, spell lists, and much more!
In Races & Cultures: Underground Races you get:
A full write-up on every one of the underground races presented in Creatures & Monsters (a total of 13 races with more than half a dozen pages on each race).
Each write-up contains detailed information about religion, style of clothing, weaponry, armament, lifestyles, and more.
Training packages unique to members of a given races that represent special kinds of lifestyles and vocations found only among members of the race.
Spell lists available only to members of a given race that represent special magical abilities of certain races.
Full adolescent development tables that are completely compatible with the RMSS.