If you are already working with some finite-state toolbox (such as the FSM tools from AT&T [Mohri, Pereira, and Riley 1998] or van Noord and Gerdemann's [2001b] FSA utils), then this book will provide insight into the vast possibilities that the technology offers and will help you place your own work in context.
if you already work with LEXC or XFST, this is the bible of the applications.