There are 2 places I know it’s linked from, but both of them are hidden within a tooltip. I’m not sure there’s a link to it anywhere that’s more obvious.
The book providers page is such a great resource. A couple of comments and updates:
Bookwalker accepts PayPal. This can be helpful for readers outside Japan who run into problems getting their credit cards accepted. (I used to use Honto.jp exclusively, but recently have become unable to make new purchases there due to problems that came up when I tried to update an existing credit card’s expiration date.)
It is possible to use the iOS (iPad and iPhone) app for Honto.jp’s ebooks outside Japan. You have to set up an Apple ID with Apple Japan, enter that Apple ID in iOS (Settings > Apple Account > Media & Purchases) and then open the App Store. That will enable you to download the Honto app. You don’t need to change the main Apple ID you’re using on your iOS device, just the one for Media & Purchases.
The Honto app on iOS lets you easily look words up in the free dictionaries (e.g. Japanese and Japanese-English) which are accessible through the iOS operating system, or on the web. Honto’s books through the web browser do not support copy and paste or dictionary lookup.