It has been announced that XXXHOLiC will be adapted into Novel written by Nishio Ishin. This author has a rather creepy name, no matter if you read it front to back or back to front, you’ll always get “Nishio Ishin” (it’s probably a pen name, though).

The novel’s title will be “XXXHOLiC　アナザーホリック　ランドルト環エアロゾル” (XXXHOLiC – Another Holic Landolt Ring Aerosol), and the first chapter will be published on Kodansha’s new magazine “Comic Faust“. CLAMP will draw the title page for this chapter.

The magazine is out on June 15th, while the complete Novel is scheduled to be released on August 1st as well as Death Note’s Novel, which will also be written by Nishio Ishin. Price: 1365 yens (both magazine and novel).

Interesting, huh? More info (japanese) here.
EDIT:
Tsubasa & XXXHOLiC Spoilers