Some of the Islamic prisoners like HP a lot. Nobody should tell the US government about the political content of book 6.
Nitpick: The article mentions prisoners getting "translations" of the Koran. As I understand it, translations are permitted but considered non-canonical. I suspect the word the WashPost wanted was "transcription".
As for conditions at Gitmo, I notice that there was no mention of what proportion of prisoners have access to books.
Links found at the exemplary Geekpress.