The download Button or option on OkadaBooks is supposed to download your bought books for you to read offline with your data turned off on the app alone.
While downloading, if the book:
Cancels: Check your network - Turn off data and turn it back on and tap (or press your finger on the book) for a few seconds, the download option will appear, click on Download to start downloading the book all over.
Paused Download: Check your network - Tap (or press your finger on the book) for a few seconds, the download/Cancel option will appear, click on Cancel to cancel the download or click on Download to start downloading the book all over.
It sounds all difficult at first, I know. But it all just gets solved for you at the click of a button.
No, most of the books are less than 1mb so a large chunk of your data will not be used
Finally, You won't have to do this all the time. Have fun