From the introduction:
Astonishing Races: Leprechaun gives the leprechaun a full-racial review and expansion.
Astonishing Races: Leprechaun aims to give players of the lovable, gnome-like tricksters immersive, world-neutral materials and support for almost any kind of play. Included are new and expanded options for alternate racial traits and a leprechaun-blooded gnome, favored class options for all the base, core, hybrid, and occult classes, and 3 racial archetypes. The book also has new racial rules for leprechaun equipment, feats, traits, magic items, spells and more!
This major overhaul of Astonishing Races: Leprechaun roughly doubles the amount of information available.