I visited the Taj Mahal and the soldier who searched our bags found that I was carrying a book about Africa. She flicked through it and then said that it was not allowed in. Indeed, we noticed (too late!) that there are signs at both the Taj Mahal and Agra fort specifically prohibiting books.
However, I was not able to find out why this is. The website only says that "big books and bags" are prohibited, but this book was a small paperback.
Also, the guard at Agra Fort found the book but allowed it in, although they seem to have the same list of prohibitions.
I wonder whether it is something to do with the Taj Mahal being Islamic (the book had pictures) or is there a different reason?