Alert guests to the bring-a-book theme using the baby shower invitations. Use invitations that look like retro library check-out cards or create invitations by arranging quotes from classic children's books around the edges of the stationary. Include wording such as, "To help Beth and Bob pass their love of reading onto Baby Smith, please bring one of your favorite story books to add to their library."
It's helpful to include a small note on the invitations asking guests to bring along gift receipts for their books, in the event that multiple people bring the same books the parents-to-be already have.
Extend the book theme to the rest of the shower. Arrange children's books at the center of each table to serve as centerpieces. Order or make a cake that looks like a pile of books. Stack several rectangular cake layers at slightly different angles and frost each layer to look like a different book.
Give bookmarks as favors, or purchase small journals to give out and wrap them in pages from old books. Scour a used book store for damaged books to take apart rather than cutting up books that are still usable.
If attending a bring-a-book baby shower, purchase a large decorative basket or small, sturdy bookshelf as a gift that the couple can use to hold their new books.