Since we had only seen a very small sample of the cover, I was under the impression that Nekoi could have drawn it, but now you can clearly see it's Mokona. I really like when CLAMP draws for a novel, it's a chance to see new styles of their art, this one is so gorgeous, reminds me a bit of CLOVER.
CLAMP also drew the cover for Sanshou Dayuu/Takasebune/Abe Ichizoku's pocket edition,
If you find more pictures of these please let me know! xD