Amazon offers Magic Tree House Merlin Missions Books 1-25 Boxed Set for with free shipping.

Product Description:
Merlin Mission Books 1–25 of the #1 New York Times bestselling Magic Tree House series boxed together for the first time ever!
 
Magic Tree House Merlin Missions are more challenging adventures for the experienced Magic Tree House reader. In each adventure, Merlin the magician sends Jack and Annie on an adventure in the magic tree house.