From Muppet Wiki

A band of pirates calling themselves the Bookaneers are led by their scurvy captain in Sesame Street Episode 4135. She leads an Anything Muppet and a salty dog on a quest to Hooper's Store to obtain a book being read by Elmo and Alan. When Elmo expresses a shared interest in reading, the captain invites him to join their group, but only if he can pass several tests.

After a treasure hunt that leads them to the Sesame Street Library, she reveals that their hunt was for the treasures to be found in the pages of books stacked on every shelf.

There is also a stage show at SeaWorld Orlando called Elmo and the Bookaneers, which debuted a few months before the episode.