If you've bought any ebooks from O'Reilly then you probably got an email this morning.
They're beta testing a new feature called "Sync Ebooks to Dropbox" , and assuming it keeps working the way it is supposed to you're going to be able to sync ebooks bought at O'Reiily to an account with Dropbox. You need to have an account first, but once you set up the connection the process should work automatically.
As cool as this feature is, I think I'll stick with my current method.
I prefer to download the ebooks and add them to my calibre library. This lets me control the metadata, available formats, and generally manage my ebook library in a single location. But that doesn't give me an online backup, which is what O'Reilly offers above. But that's okay because I also have the Dropbox app installed on my laptop. It's set to sync my calibre library folder with my Dropbox account.
It takes a little more work but that's a better option, don't you think?