There are many aspects he may have enjoyed about the Magic Tree House series – the action, suspense, and/or time-travel elements, etc. These elements can also be found in slightly more challenging material, too!
If the adventure and history appealed to him, check out The 39 Clues series, the first of which, The Maze of Bones, is written by Rick Riordan. Not only does this series introduce the reader to lots of super-cool history, but it also offers up a dramatic family rivalry, with sibling annoyance included.
If he really enjoyed the time-travel aspect, he may also enjoy the Time Warp Trio series by Jon Scieszka, about three boys with a magical book that transports them through time whenever they open it. This series isn’t necessarily more challenging, but it would give him a chance to try something new.
A slightly more advanced series is The Sixty-Eight Rooms, in which two sixth graders use a magic key at the Art Institute of Chicago to travel to various time periods and places.
Recommendations based on other elements in the Magic Tree House Series:
Guardians of Ga’Hoole by Kathryn Lasky – action-packed fantasy series.
Inkheart by Cornelia Funke – magic and reading mix easily in this series as our 12 year old protagonist learns just what can happen when fictional characters come to life.
Starcatchers series by Dave Barry – While this series is quite a bit more advanced, especially in length, it would be a great choice for a family read or one for a long trip – like coming back from a holiday vacation.
Any books by Kate DiCamillo, though not a series, may be a great fit, too. The Tale of Despereaux follows a mouse on a journey of epic proportions, The Magician’s Elephant offers a story of love, sorrow and wonder when a magic trick goes right and disrupts the lives of all involved, and The Miraculous Journey of Edward Tulane introduces us to a snobby toy rabbit whose life will never be the same after he falls overboard, becomes lost at sea and begins the journey of his life.
Another place to look for book series recommendations is the Novelist K-8 database on the library’s website. When you enter a title or author of a book or a series that you know your child enjoys, you can click on the option for read-alikes to get recommendations.