eBooks purchased with an Apple ID using the old iOS app (BOOK ☆ WALKER 1G / ver.2.3.8) have disappeared.
You can read your purchased eBooks by transferring the [Purchase History] from old iOS app (BOOK☆WALKER 1G).
eBooks purchased using old iOS app (BOOK☆WALKER 1G) without registered as a member of BOOK☆WALKER will not be displayed temporarily after signing-in with the current app.
It will be displayed by transferring [Purchase History] from the old iOS app.
How to transfer [Purchase History]
※The BOOK ☆ WALKER 1G app is not available on iOS 11 or later. Please use it on iOS 10 or lower.