This month brings to a close a favorite book series among young children and young-at-heart readers everywhere. Mo Willems’ tales of Elephant and Piggie come to an end with The Thank You Book, the 25th published adventure of Gerald, the thoughtful, worrying Elephant, and Piggie, his exuberant and optimistic best friend.

Since the 2007 release of Today I Will Fly, the Elephant and Piggie books have garnered rave reviews, won two Theodor Seuss Geisel Medals, received three Geisel Honors, and spawned a musical! This summer, fans also have an opportunity to view the Art and Whimsy of Mo Willems, an exhibition at the New-York Historical Society Museum & Library. Running until Sept. 25, the exhibit features original art, sketches, and inspirational drawings from the author’s most popular series, as well as stand-alone classics.

We think you can’t go wrong with any of the Elephant and Piggie titles, all of which are available through BCLS. Here are ten of our favorites:

In There Is a Bird on Your Head!, Gerald discovers that there is something worse than a bird on your head-two birds on your head! Can Piggie help her best friend?

In I Broke My Trunk! Gerald tells Piggie the long, crazy story about breaking his trunk. Will Piggie end up with a long, crazy story of her own?

In Pigs Make Me Sneeze!, Gerald believes he is allergic to his best friend! Will he have to stay away from Piggie forever?

In Can I Play Too? Gerald and Piggie meet a new snake friend who wants to join in a game of catch. But don't you need arms to catch?

In Watch Me Throw the Ball!, Gerald is determined to teach Piggie that ball-throwing is serious business... but Piggie is just as determined to have serious fun.

In We Are in a Book! Gerald and Piggie discover the joy of being read. But what will happen when the book ends?

In I Really Like Slop!, Piggie invites Gerald to try her favorite food . . . slop. But Gerald is not so sure he's going to like it. At all.

In My New Friend Is So Fun!, Piggie has found a new friend! But is Gerald ready to share?

In Let’s Go for a Drive! Gerald and Piggie want to hit the road! But the best-laid plans of pigs and elephants often go awry.

In Are You Ready to Play Outside?, Piggie can't wait to go play in the sunshine. But will a rainy day ruin all the fun?

What’s your favorite Elephant and Piggie book?