I have a large amount of books on my Bookwalker watchlist (click on the grey checkmark to add one, and click on the checkmark next to the search bar to look at your list). Whenever one of those books is discounted, I get a filter link above the books so I can look at those that match the discount. Discounts are either price reductions or increased coins (cashback) so you can see either or both at any given time, depending on which one applies.
Amazon Japan lists the “discount” percentage if you add the book to your wishlist. For example, I added ビブリア古書堂の事件手帖 ~栞子さんと奇妙な客人たち~ in February and it shows me that the price has decreased 9% since I added it to my wishlist, and it was originally ¥643 when I added it. However, on the main book page you will see that ¥584 is now listed as the Kindle price with no indication that the price dropped (or whether it was a discount vs permanent change). I also don’t know if there is an option where they will email you when there is a price drop.
The one thing I wish they would add to the wishlist is the points back percentage. To me that is basically a sale since I will definitely use the points for more books, but I haven’t found an easy way to tell outside of clicking on each book (which is how I currently check).
I just discovered (thanks to the Bookwalker Freebies thread) that there is a manga adaptation of this book, and the first three volumes are currently free to keep (until Jan 20th). In case you are interested: 獣の奏者(1) - マンガ(漫画) 上橋菜穂子/武本糸会(月刊少年シリウス):電子書籍試し読み無料 - BOOK☆WALKER -