Owl

Pretty sure you need to use the reader. I don't understand what you mean about the mail and bookwalker, but I use bookwalker and it is good.

They told me I can't use the same email for login at cdjapan ebooks purchase and bookwalker, as cdjapan app is made from bookwalker app, so there will be login problems.

Aha, so should I use bookwalker instead?

Somehow I'm afraid I have problems because of reginal restrictions. For ex. I can't buy VIZ media ebooks from amazon, because of restrictions.

Yeah there are some restrictions. Depends on the title 日本??定 are Japan only for instance.