ASCII Media Works
A 160-page guide book titled was published on December 10, 2005 by ASCII Media Works. Two more guide books were published by ASCII Media Works for the anime adaptations: on October 27, 2006 with 176 pages, and on October 19, 2009 with 168 pages. Four 128-page art books illustrated by Ito were published by ASCII Media Works: on February 25, 2005, on August 9, 2007, on August 10, 2009, and on August 9, 2013.