Note 2: The migration process has closed in May 2020 (or so).
Note 3: Japanese mobile numbers are now required if you wish to create a Yahoo! Japan account 😦 – I recommend checking BookWalker for international-friendly accessibility for manga in Japanese and English (global store).
I will go into detail about the merging of eBookJapan with Yahoo! Japan’s Bookstore (their ebook retail arm). Read it here.
This post is originally published on September 21, 2017 with major revisions made on June 13, 2019. Some revisions are still ongoing.
Page 1 This page is for existing users. Read more after the cut.
Page 2 for creating a new account.
Page 3 is how to login on Android.
Page 4 is the out-of-date guide for the now defunct eBookJapan Initiative website.
Note: This post is heavy on graphics/screenshots.