It's a decent guide in English. There are a ton of resources for hiking in Japanese, but English resources are quite lacking. This books gives a good overview of probably the most popular hikes in each region and although I haven't had the opportunity to go to all these places, since I live here in Japan, I will keep this on my shelf to refer to in the future. If you can read Japanese, I would recommend the ~の山 series of books, which has probably the best hikes broken down into volumes for each prefecture. Alternatively there are quite a few blogs of avid hikers in Japan that write in English as well.