This is a book made by the developers for fans of Xenogears that tends to get cited a lot. It has art and illustrations in color and sketches of character designs for almost every character, along with cities, townspeople, and Gears. A lot of the book is devoted to in-depth information about the game's history, characters, science, world, and story. Soraya Saga made a story called Dark Summer which is featured in it and there's a comic about the Elements girls at the end of the book.
This study guide have no plans to translate this book, but you can find other peoples translations of this book if you search on the net. What this page offers is a second opinion on some of the translations.
Translated by Gwendal, Jinx and others