Castle library is a possibility (I was under the impression it had books from many worlds) so anything on ancient myths, Gods, or x-ref'd against the real world Mara (it won't be able to help announcing what it is, it's quite proud of the fact).

For in his room, there'd be books in the TARDIS library; however the Mara has a habit in the past of destroying info about itself - it'd be easy to say though, that there's partially destroyed books in his room and it wouldn't be a stretch to say they could be repaired by certain magical individuals, perhaps? What do you think?