In this context, "a pop-up" means one of those books that is usually for children where the pictures inside the book "pop-up" when you open the book.
A pop-up doesn't have to be a children's book; and there are some pretty amazing pop-up books out there. But because of the context in your sentence, it probably means a children's book with pop-up pictures.
Here's an example: http://www.bergintoys.com/heros/2004...%20Book%20.jpg
It's a pretty funny sentence, I'd say.
- For Teachers