May I include anything with my book?

Generally: No.

Media mail regulations are very strict: only media (books, CDs, DVDs, VHS tapes) and an invoice (page 2 of the PBS wrapper qualifies as an invoice) are allowed to be sent using Media Mail rate.  No advertising, commercial solicitation, political materials, religious tracts, non-Media items, letters, cards are allowed.

PaperBackSwap rules preclude including extraneous materials such as advertising, commercial solicitation, political materials, religious tracts, non-Media items, letters or cards. Including inappropriate materials with your PaperBackSwap package (whether sent Media Mail or First Class Mail or any other postal rate) will jeopardize your membership and can result in termination, even after a first offense.

I recently received a fifo book from a stranger here that included a 2 page religious article that had been photocopied from a magazine. I kindly let the sender know the above rules.