I download from DropBox too. In fact, I keep my Calibre library in DropBox, and this works very well for me. So: add new book to Calibre, the DropBox client automatically uploads it to the DropBox cloud. The new book is now available at any time for me to download to Marvin.
Warning: you have to be careful to make sure Calibre is not running before accessing the library in DropBox from elsewhere. This is OK for me because I only run Calibre to add new books to my library.
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