It seems like Google is trying to phase out the personal library functionality of the old Google Books in favor of Google Play Books. Now, Google Play Books is pretty nice. However, if you’re like me, then you uploaded a hoard of PDFs to the old module which was especially nice since it didn’t count towards your Google Drive storage.
I found out recently that using standard download instructions, you can not download the old books (or even open them in some cases). This was both frustrating and disapointing both for the mobile and desktop variants.
Fortunately, a bit of Googling landed me at the below link which worked like a charm. You can archive all of your Play Book (or any / all of your Google account information) and have it emailed to you or added to an assortment of cloud storage services. You can then download and unzip / untar the content.