The Demigod Diaries by Rick Riordan

Although there's a quiz and a word search, this book mostly contains short stories: there's one starring Percy and Annabeth between the Greek and Roman series, one featuring Luke before he came to Camp Half-Blood, one starring Leo making Festus the dragon into the Argo II, and ends with one written by Rick's son Haley about what happens to a demi-god who fought for Kronos after their defeat. I enjoyed all of the stories--they are good light reading, and they helped put me in the mood for The Mark of Athena, which comes out in October. Haley's story is just as good as his dad's.