The Magic Tree House series is a series written by Mary Pope Osborne, which started in 1992. Sal Murdocca illustarted every single book. Since, then, there have been 40 books published. There are seventeen Magic Tree House Reserach Guides to act as non-fiction companions to only some books. William Osborne, Mary's husband, is a co-author of all the research guides.
The series starts with book #1, Dinosaurs Before Dark. In that book, 8-year old Jack, and his seven year old sister Annie come across an old treehouse, and find that it is stacked with books. They accidentally say the line "I wish I could go there" which sends them to the picture in the book they are pointing at.
So far, they're have been 42 fiction books named, and 19 research guides planned. Their titles are listed below.