[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 ...
The Magic Tree House books are a great choice for language learners at a low level. The stories are fun, and there's a great deal of variety in subject matter depending on which book you choose: ghosts, dinosaurs, dragons, historical events, natural disasters, etc. At the end of the book, there's a brief article discussing the real life inspirations for the book's story — for example, in this book, it discussed the beliefs of ancient Celts regarding crows. There's absolutely no need to read the series in order, the book explains the premise and any necessary background info in a prologue.
Language-wise, as with most books in this age group (6-10), all kanji have furigana. Many words are written with hiragana alone. Text size is larger than normal, and the book is fairly short, only about 150 pages with some illustrations. The sentences, reflecting the original books in English, tend to be simple, direct, and concrete so they're not too difficult to parse, though there is quite a variety of vocab to look up due to the historical setting and subject matter. Merlin has a little bit of old-man dialect going on, but otherwise there was no keigo or slang.
(FYI, in English, this book is called Haunted Castle on Hallows Eve.)