My middle son first discovered this series when he was 5, just when he was starting to read. As he progressed from being read to, to reading independently, he never once lost interest in the series. In each of the volumes, siblings Jack and Annie embark on various adventures through a magical tree house that appears in their back woods. The tree house sends them back in time where they solve riddles, perform other important tasks, and most importantly – learn history.
The writing is perfect for this age group, and each book is educational (there is also some companion research guides for many of the books in the series). Some parents may be a little wary of the books because of the “magic” involved. With the exception of a few titles (mostly the “Merlin” missions), I never sensed the magic overpowered the learning, or spoiled the fun of the read. The magic is just the vehicle used to transport the kids back in time, and is a convenient tool to get them out of a tight spot. When we did run across something that was a little weird, we discussed it. Though I’d like to think I had the most influence, I have to give the Magic Tree House books some of the credit for turning my son into a voracious, enthusiastic, and very confident reader.
Related website: http://www.marypopeosborne.com/