My son has been reading the Mary Pope Osborne Magic Treehouse series for over a year now. I absolutely love this series when looking at content. Some of the mechanics/sentence structure--not so much. Another reason I love these books is because many of the topics have a companion book like a non-fiction research guide. These are a little more difficult to find, but worth it. Depending on the site you follow, the reading level varies from first to fifth grade. They are great "first chapter books." They are also great read-alouds. Our first grade teachers read these to the class.