I work as a librarian in a small international school in Suriname, South America. We have several of the Magic Tree House books and I can hardly keep them on the shelf! These books have nice illustrations and the reading level is somewhere around 3rd or 4th grade, but younger students often check them out to have a parent read to them.
The main characters, Annie and Jack, are sister and brother who travel through their tree house to places such as the Old West or Camelot to complete some task given to them in rhyming clues. Through the course of their adventures, they learn lessons about courage, friendship, loyalty, and trust.
These books are fun and easy to read and are great for kids who like action and adventure.