Anna is a young girl who has left her foster parents and has been left in the care of an elderly couple in a seaside village. She makes friends with a mysterious girl of the same age, Marnie, and they play together every day. However, no one in the village knows about Marnie.
If you like the movie you will like the book
When I re-watched the movie this January, I first thought about reading the book. Then when I finally had it in hand I wondered if it would be as easy as I hoped, as I saw it was not a full furigana book. But after starting, it turned out that only the easy kanji were without furigana and the text was in fact kanji-sparce, the way 魔女の宅急便 is.
This made the text far easier than I feared. But as I understand this might not be the case for more kanji-affine learners. There are, of course some more difficult passages, where I doubted that what I read was actually meant. But all in all it was the easiest novel I read so far, with an average of just 3.6 look-ups (grammar + words together) per page. I will probably buy some more books in the 岩波少年文庫 series.
In contrast to the movie, which is a Japanese localization, the book is a translation of the original English text. And the original book is placed at really existing places at the coast of Norfolk, England. After some re-transcribing of the katakana town names, I actually found the place in google maps ;-)
The first half of the movie contains about the first book's contents, which means the book is far more detailed than the movie, but the movie is a very good summary of it.
I would say, if you like the movie, you will probably also like the book. On the other hand as a book intended for >= 10-12 year olds, it may not be everyone's taste.