I use Calibre on my desktop to manage my library From there I drag and drop onto Google Books in my desktop to upload. My library is 15GB so cloud is not really an option. Google Books only supports EPUB and PDF. Calibre can do the conversion from Mobi to EPUB if they are not DRM'd
OP can correct me if I misunderstood but I think he's talking about multiple devices as well ? eg I read an epub book on my android phone on google books, I open it via the Google Books App on Chrome on my SP3 and it opens onto the same page, which is a feature that's really convenient.
Except more than 1/2 the time I don't get the handles at all, so I have to select another word, then come back and hope I get the handles next time
Same I I triple tap, it selects the sample but I don't get the handles.
Makes selecting text tedious.