[DeepL Translation - needs review] Introduction to Volume 1: Jack and Annie are good friends who live in Pennsylvania, USA. One day, they found a mysterious treehouse in the forest on top of a big oak tree. While looking at a book about dinosaurs inside, the treehouse suddenly starts to spin around and around, and before they know it, they have wandered into the real age of dinosaurs. The treehouse turned out to be a magical time machine that allowed him to cross time and space to the world ...
If you're only interested in the Magic Tree House books for their utility as a language learning tool, this entry in the series may be of particular interest to you. Since it is set in Japan during the Edo period, you can pick up a variety of cultural and historical vocabulary concerning food, samurai, sumo, haiku, and so forth. This of course will mean a lot of dictionary lookups, and there is a sprinkling of unusual dialects (samurai talk, old man talk, etc.), but other than that it's as easy as any other book in the series and a lot of fun.
In English this book is titled Dragon of the Red Dawn. However, according to the afterword it appears this book was to some extent rewritten for a Japanese audience — so I don't know that having an English copy of the book would be useful for reference.
If you're looking for the other Magic Tree House book set in Japan (Night of the Ninjas), it was combined with another short Magic Tree House book (Afternoon on the Amazon) and published under the title アマゾン大脱出.