I'm sure there is a better way to do it, but I just have a folder on my Google Drive
and use right click and "Save to Disk" -> "Save only epub format to disk in single directory" That is pretty much the only directory I ever save to disk in, so it remembers it.
On my tablet, I can open from the GDrive app, and it'll suggest the Google Play Books app as the default but you can open them in any other reader you have installed too.
The only other way I know to get a book into the Play Books app is to use it's upload feature, which is super fiddly. Although, if there's a way to automate this or do it easier, I'd be happy to hear it.
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