The book in question is an Arabic translation of Chomsky's Interventions, an anthology of op-ed pieces written for the New York Times Syndicate (as the home page for the book points out, while "New York Times Syndicate writings are widely published around the world, they have rarely been printed in major U.S. media; none have been published in the New York Times.").
The reasons for the ban? The article continues:
Prison camp officials would not say specifically why the book was rejected but Navy Lt. Cmdr. Brook DeWalt, a Guantánamo spokesman, said staff reviews "every proposed or recommended library item to assess force protection issues associated with camp dynamics -- such as impact on good order and discipline.''
A rejection slip accompanying the Chomsky book did not explain the reason but listed categories of restricted literature to include those espousing "Anti-American, Anti-Semitic, Anti-Western'' ideology, literature on "military topics,'' and works that portray "excessive graphic violence'' and "sexual dysfunctions.''
I can't say that I find it surprising: U.S. foreign policy includes lots of graphic violence, and if not literally sexual dysfunctions, at least institutionalized perversion. I guess that means that the prisoners at the 'supposed to soon be closing camp' will have to stick to Harry Potter and Richard Nixon.