There was hope when the iPad was released that it would act as a kurofune for the Japanese publishing industry[0], but that of course has not come to pass. However, I have found a surprising number of books on the Japanese Kindle store as of late.
Yeah, there are more, but still the most successful authors like 東野けいご are nowhere to be found, and authors like him sells millions of books every year.
0: http://www.cnet.com/news/apple-appeal-in-japan-is-turning-th...