XXXHolic is a supernatural/comedy themed manga/anime that is apart of the CLAMP series. It is one of the more popular of the CLAMP series and often shows other CLAMP series being mentioned (ex. Chobits, Sakura Chronicles).
It revolves around a boy named Watanuki and how he stumbled upon a shop owned by a woman named Yuuko. Yuuko's power is to grant wishes, but she can only do so with proper payment. Since Watanuki has seen spirits ever since he was born, Yuuko offers to take away his sight of the spirits in exchange for him working at the shop. He agrees and works at the shop until his debt is repaid.
(Source: xxxHolic Wikia)
The series was re-named to xxxHolic•Rou on the title pages of each chapter from chapter 186 and on, but remains ×××Holic on the front covers, spines and indicias of the corresponding Japanese volumes (16 and 17).
A oneshot published in May 2010 to commemorate the switch was later collected into Volume 18.