According to Comic Natalie, the XXXHOLiC Artbook will be released on the Japanese summer. The artbook will be entitled XXXHOLiC Gouka Birei Gengashuu (『XXXHOLiC』豪華美麗原画集), which can be roughly translated as XXXHOLiC: The Gorgeous, Beautiful Artbook.
Before the artbook release, a Postcard Book will be released in May. The editorial office of Young Magazine seems to enjoy Postcard Books, I think. Chobits also had one.
There are a lot of XXXHOLiC goods to be released for the next months, here is a summary of what’s coming:
March 9: XXXHOLiC #19 (along with limited edition OAD).
March 9: XXXHOLiC Guidebook.
May: XXXHOLiC Postcard Book.
Summer: XXXHOLiC Artbook.