Jack and Annie are brother and sister. One day, they go out exploring and come across a treehouse that is filled with books. They quickly discover that if they point to a picture in a book and say, "I want to go there!" they will suddenly be whisked away on a magical adventure. They usually visit different times in history, to witness and participate in some kind of famous historical event. Jack always takes careful notes in his notebook and is the cautious one. Annie is notorious for getting herself into trouble and being overly friendly.
Sometimes the children are given a particular quest. These quests will span four or so books, where they have to locate certain objects on their travels. These objects are clues to a mystery or can break a spell.
The earlier books, while they have great storylines, often make me crazy to read. Sentences are broken up into multiple phrases and are grammatically incorrect. It looks like it was done to aid the reading process, but still is not okay. The advantage to reading these out loud is that I can blend these phrases together to sound proper again. Because of the children's adoration of them, though, I am willing to forgive her.