Chrono Cross: Missing Piece
Chrono Cross: Missing Piece (“Missing Piece”―クロノ・クロス設定資料集?) (full name: "Missing Piece" Creation of Chrono Cross Document Collection) is an official book concerning Chrono Cross, published exclusively in Japan by DigiCube. It documents a various amount of non-gameplay related information on Chrono Cross, such as artwork and illustrations, profiles for all playable characters and many non-playable characters, concept art, storyboards for the CG movies, as well as an exclusive interview with the main character designer of Chrono Cross, Nobuteru Yūki.
There is some plot information in this book that is not provided anywhere else - including the game itself and the Chrono Cross Ultimania guide book. As it is an official release by Square, most plot information in this book can probably be treated as canon, but much like Ultimania, there may be some information that conflicts with what Chrono creators such as Masato Kato have said.