Chrono Cross Missing Piece
Missing Piece is a large art book released by Digicube for Chrono Cross. It featured much location art, character and monster sketches, and other curiosities, in addition to full character art for every person in the game. Also of note are sketches of characters in their development stages (including a Guile meant to resemble Magus moreso), sketches of unused characters, and an interview with the art team behind the game. Sadly, the full character art at the beginning is often obscured a little by Japanese characters or information boxes.
Except for the interviews, all Missing Piece's relevant art can be found at Official Art (Chrono Cross). Relevant translations can be found at Supporting Material Translation. (Most of the information is basic stuff already explained in-game.)
We have some other higher-res scans than those commonly found on the Internet as well: