Shinto by Paula R. Hartz is a great introduction and overview of the ancient home-grown religion of Japan. One of the major animists religion in the world, Shinto is driven by the belief there are gods in everything. It’s also a key driver in Japanese culture.
What’s especially helpful with this book is so many folks heading to Japan are going to be going to visit a shrine (and Buddhist temples – this helps sort the difference).
This short volume offers not only the background of Shinto beliefs, but a glimpse into the story behind the shrines themselves.