I have my books converted to ePub and mobi in Calibre, and the Calibre library is synced to Dropbox.
Then whenever I want to read a book, I open the Dropbox app on my iPad or iPhone and open the ePub in FBReader. The ePub syncs between both seamlessly. You can also use iBooks this way (I used to use that, but switched to FBReader a while back).
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