I found the way to restore! There was a tiny restore button at the bottom of the purchase screen. And I can read all the books again!!
購入画面の一番下にrestore ボタンをみつけて解決しました!
↓↓↓
I had already bought this Full version, but suddenly I could not see any book.
私も購入していたのに見れなくなりました。改善してください。