By the way, this discussion might perhaps inspire Kris to do something I know he wants to avoid: integrating Dropbox into Marvin on a deeper level.
To me personally, this feature has zero importance, and there are likely lots more important features Kris might wish to work on instead, but I can imagine a Marvin user like Rhialto would love such deep Dropbox integration in Marvin.
It's probably a futuristic concept, and likely not made possible by Dropbox as of today, but wouldn't it be great if we could simply browse Dropbox folders from within Marvin, while the books would be displayed in a similarly smart way as they are displaying when we browse, say, an OPDS catalogue in Marvin?
Or think of this: when you browse your folders with photographs in your Dropbox app, you automatically get to see the thumbnails of all photographs. Wouldn't it be great if, while browsing Dropbox folders with e-books in them, we'd automatically get to see the thumbnails of all the book covers? And if clicking on such a thumbnail of a book cover would display that book's metadata and description?
Also, it would be great if we could
virtually sort books present in our Dropbox folders: for example, we might pick the "Sort by series" sorting option in Marvin, and Marvin would sort all the e-books from Dropbox using that particular criterion -- while the e-books would still remain stored not locally on the iPad, but in the original Dropbox/Calibre folders. I'm afraid this isn't currently possible, because there is, I guess, no way to extract a book's metadata and book cover from Dropbox, without downloading the entire book.
So none of this will likely be possible for Marvin to accomplish without Dropbox meeting Marvin half-way, so to say, by extending the current Dropbox functionality.
If that worked, though, it would be the best of both worlds. For a Marvin user like Rhialto, the difference between having a book stored in the cloud, and having it stored locally, would pretty much disappear. Downloading a typical e-book takes only a few seconds, after all, so there would be no reason to permanently store our e-books in Marvin. For this, though, like Rhialto said, the display of books stored in Dropbox folders would need to be similarly smart as it is now in Marvin's internal Library.
A related (futuristic?) feature wish I might submit: not sure if this is technically possible, but I would welcome if Marvin indicated to us, while we're browsing the Drobpox listing of books, or an OPDS catalog, that we already have a particular e-book stored in Marvin at that moment. There could be a check-mark shown next to a book title, or something similar, telling us we don't need to bother downloading that particular book into Marvin, because it's already there in Marvin's Library.