From Tokyopop:
To honor CLAMP's 15th anniversary, CLAMP no Kiseki: The Ultimate Collection features full-color pages, exclusive material, 6 pages of new, never-before-seen manga, and limited-edition collectible figures!

Each volume is approximately 32 pages in total, most of which features art [some of which you would have seen before] concerning the series included in the volume and other information. There are approximately 6 pages in each volume that features a new short manga, that is otherwise unrelated.

Volume 1
Cardcaptor Sakura
Chess Pieces: Sakura Kinomoto, pawns x2

Volume 2
Cardcaptor Sakura
Clover
The One I Love
Chess Pieces: Tomoyo Daidoji, Kimihiro Watanuki, pawn

Volume 3
Tokyo Babylon
Chess Pieces: Subaru Sumeragi, pawns x2

Volume 4
Magic Knight Rayearth
Angelic Layer
Chess Pieces: Misaki Suzuhara, Seishiro Sakurazuka, pawn

Volume 5
Clamp School Detectives
Duklyon: Clamp School Defenders
Man of Many Faces
Chess Pieces: Su, Nokoru Imonoyama, pawn

Volume 6
RG Veda
Chess Pieces: Hikaru Shidou, Ashura, pawn

Volume 7
Chobits
Wish
Suki: A Like Story
Chess Pieces: Chi, Akira Ijyuin, Mokona Modoki

Volume 8
X Chess Pieces: Kaumi Shiro, pawns x2

Volume 9
X Miyuki-chan in Wonderland
Shirahime-Syo: Snow Goddess Tales
Chess Pieces: Fuma Mono, Miyuki-chan, pawn

Volume 10
xxxHolic
Legal Drug
Chess Pieces: Fai, Yuko, Ichihara, pawn

Volume 11
Tsubasa: Reservoir Chronicle
The Legend of Chun Hyang
Chess Pieces: Kohaku, Kurogane, pawn

Volume 12
Tsubasa: Reservoir Chronicle
Chess Pieces: Syaoran, pawns x2