Recommended for: Ages 7 and 8
In books 5-8, Jack and Annie's friend, Morgan le Fay, is in trouble They must find four "M" things to free her from a spell. Their adventures take them to meet a ninja master in ancient Japan, flee a crocodile on the Amazon River, discover the cave people of the Ice Age, and blast off to the moon in the future. It's a difficult and dangerous mission, but Jack and Annie will do anything to save a friend.
When I was about seven, this was my favorite series. I rarely reread books but I read the ones that I owned multiple times. One thing that I didn't like about this series was the evolution, but I would just ignore it. In this series Jack and Annie, brother and sister, would travel through time in a tree house visiting places all over the world. They would visit famous people, like Shakespeare, and experience well-known events such as Pompeii and the Civil War. Sometimes they would have to search for things. Later there were Merlin missions but I only read two of them. This series is the reason that I think of Morgan le Fay, from the King Arthur legends, as good. I was surprised to find out that she is usually a villain. In later books there is more about the Arthur legends but I don't remember reading them. One thing about the time travel in this series that makes it different from most time travel, is that they don't have to fix history or mess up history and then try to fix the problems they created. The way they travel in time is not scientific. All they do is find a book and point to the place and say "I want to go there". They would then use that book for information about the time and place that they traveled to. Sometimes they would end up in the jungle or in a savannah in Africa. The main character is Jack but Annie was my favorite. I don't think they got along perfectly but I don't think that they had a horrible relationship.
These books have a lot of adventure and I don't remember them being scary at all, but some of them might have been. I loved these books and I still loves stories about time travelers in a wooden box. I will always remember this series fondly and I would recommend it to young readers with the one caution that there is evolution in them.