The artbooks by Clamp for X are really awesome stuff. Probably among the best artbooks I've ever seen. But some of you already know Clamp's style is so rich and beautiful, with lot of decorations and very detailed. Especially some of the arts for Kamui are something extraordinary.
I've seen scans of these artbooks around the web more than once, but I'm sorry I don't remember many links
At this moment, I can only post
this one, but I'll be back if I remember the others!