We have a 7-year-old who is just reaching the point of being able to read chapter books on her own. The Magic Tree House was on a list of recommended summer reading from the school, and came recommended from a friend, so I picked up Dinosaurs Before Dark to see if she would like it. We're up to the third book now, though not without a few battles. She read the first one entirely by herself; since them we've moved to a trading model.
The books are short. Each one has ten chapters, and in each the two main characters (Jack and Annie) visit the Magic Tree House and pick up a book. By making a wish, they are sent to another place and time, where they have a mildly frightening adventure, and then return home. A meta-plot makes itself known in the first book, and continues to build very slowly: Who is the owner of the tree house? Why are the books there? This is useful for holding adult attention if nothing else!
I think these are pretty good books for a young reader with a good basic vocabulary. The writing itself is workmanlike, and I am baffled by Osborne's heavy use of sentence fragments, but the settings are interesting. Osborne isn't afraid to throw challenging words in, but there are no more than a handful of those per volume (precipice stood out in #2, sarcophagus in #3). The individual chapters are very short, so it's easy to get through one a night, or two or three for that matter. Reading them doesn't feel like a chore, and there's a great sense of accomplishment for a new reader in being able to read a whole book. The perils of the characters are quite mild, so I would particularly recommend these for kids with an anxious nature.