From the back cover:

Caste Book: Twilight is the third Caste Book for Exalted - books detailing the different castes of the Solar Exalted. Within its pages are all the secrets of the Twilight Caste, from its members' unmatched mastery of sorcery to their fragmentary memories of the First Age. The book contains new Charms, rules, spells and artifacts to help them reclaim their heritage of power.