Not just the fact that this is a kids book.
The fact that is pretty much a magic/wizards/witchcraft/blah blah blah book that would probably have them being executed for reading it in their own countries.
Agatha Christie is a strange pick too, since, if I recall correctly, the target audience was women.
A lot of the Taliban are kids, or were before the Taliban brainwashed them. I've read that the Talibunnies are now reduced to impressing (drafting) 12 and 14 year olds, so maybe when those kids get to GitMo, they will really enjoy Harry Potter.