Once you buy the book, a notification comes up for a few seconds: "Go to MY BOOKS to read your book." 
This, of course, happens on the OkadaBooks App. We will talk about the website. Let's swallow this one first. So, go to MY BOOKS, tap on the book to download it. If it fails to download or anything unusual happens, tap and hold on the book for a few seconds, an option will come up "Delete book" and redownload it. Deleting the book will not erase it completely. So, you are safe. Once the book is downloaded, you can turn off your data connection before reading the book or just leave it on.

For the website
Once you buy the book, click on READ to read the book. If it says "Requires App" then the owner of the book disabled the book's sale on the website, thereby making it available on the App alone.

To combat piracy, OkadaBooks does not let you download, copy or share the contents of individual books on the platform.