Perfect Works is a book covering the storyline of Xenogears and is also linked to the storylines of the Xenosaga series. It is presumed that, had the two remained in the same canon, that the book describes how they were meant to be linked.
In addition, Perfect Works describes an enormous backstory of Xenogears, including canonical character details that never made it to the game's script.
The book has long since been out of print with no likelihood of future republication, and copies of the original Japanese book have become rare collector's items. However, the book has since been fan-translated to English.