Xenogears Perfect Works ~The Real Thing~, known simply as Perfect Works, is a book covering the storyline of Xenogears. In addition, Perfect Works describes an enormous backstory of Xenogears, including world building and canonical character details that never made it to the game's script.
Perfect Works also contains a timeline which corresponds similarly to the Xenosaga series. However, like other games in the Xeno- series, it is a separate universe from Xenosaga.
Perfect Works has been unofficially translated to English by fans.
Nice artbook, has also, in addition to pictures, all the information you need, if Xenogears' huge Iconoclastic-Soylent Green-Kabbalistic fictional world interests you.
I don't think it's an understatement to say that Xenogears is one of, if the not most ambitious game ever created. Supposedly the 5th episode out of a series of 6, this is the only was that was ever released. Tackling themes of philosophy, religion, psychology and with a story spanning over 10,000 years, it's understandable that the game would fail short of being able to explain everything going in its mythos. Time constraints and budget restrictions also made it so the Director's vision was never completed. Perfect Works, as sort of companion guide to the game, was released to try and go deeper in the story, character, as well as insights on many aspects of the game. Including a lot of of artworks, it's a pleasure to dive into it as a fan. Thanks to the power of the Internet and Xenogears' fans, this Japanese-only book was fan-translated and made accessible if you know where to look. Now, even with this book certain aspects of the story and themes remain arcane, and certain passages are long-winded and not as interesting as they could have been. Reading it with Mitsuda's OST (or the 20th anniversary orchestral concert) is a must.
This book is the ultimate for Xenogears fans. We get to see the (almost) complete story of Xenogears for the first time in one place. There are tons of images of development artwork as well as interesting interviews with various members of the development team. Do note that this is in Japanese, and I found the language level to be at a fairly advanced level. That fact didn't stop me from enjoying the book, albeit with frequent kanji lookups and many trips to the Japanese-English dictionary.
I say read. It's in Japanese. I imported this gorgeous work for the gorgeous art in it. I was not disappointed. I haven't looked at it in years though.