The odd Tradition out, the Hollow Ones didn't receive a splatbook in the initial run, possibly because they weren't recognized as an "official" Tradition by their peers on the Traditions Council -- although some would say the Hollow Ones instead had The Orphans Survival Guide.

Regardless, Tradition Book: Hollow Ones explores what isn't quite so much an association or organization as it is a mindset of iconoclasts and unique, syncretic style of magic. As a splatbook, it offers the usual components: in-character explication of the Tradition's (admittedly brief) history, beliefs and practices, sample characters, and some new magical rotes and items.