For April Fools' Day, I found this adorable book at a discount store called Garden Ridge. If you have a store by you, it is worth a look through their books. Books there are typically less than $5, usually closer to $3. I would much rather spend a few dollars on a book than close to the same amount on a card!
Anyway, this book called Lettuce In by Tina Gallo. Each page is a knock-knock joke with a door flap. Knock-knock jokes are great exposure to the concept of puns and multiple meanings. Some of the jokes are too difficult for E to fully understand, but she definitely finds humor in the ones that are based on sounds (such as "Boo who?). We have read this book at least 10 times since the first reading. She now understands the sequence of a knock-knock joke, and has memorized most of the book!