I first learned about Magic Treehouse several years ago when a student I tutored was reading them. I read them with him and found them to be fun, short novels for kids. My daughter now reads them and loves them.
What I like about Magic treehouse is how the story incorporates so many different topics, keeping up the fun plot of a brother and sister Jack and Annie, on adventures through time and space. Author Mary Pope Osborne then wrote some non-fiction companion books to provide further information on some of the topics her stories explore.
The series of books has 28, then moves on to a harder reading level, longer with smaller print at #29 when the series is called 'Merlin Missions"
Check out The Magic Treehouse Website here. There is a teacher page with resources, some games for kids and
There is a free lesson plan for some of the books available here.
A new book (#48) comes out this July with another 2 planned in the next year.
Wikipedia lists book names and summaries here.