I would like to store my books in the cloud Dropbox, Box.net etc..., view them in searchable Bookshelf and list modes ala Delicious Library and iBooks and be able to download them one at a time.
We're certainly looking into this, but the requirement of being sandboxed is being a problem for things outside of iCloud. Peter would be the final word on this, but I don't think there's anything in Bookle's data storage architecture that would make synchronization terribly hard at a basic level. Syncing changes between machines while both are in active use is probably out of scope - Bookle is meant as a single-user app, not something that a group would be using in some fashion.
Personally, I would like Bookle to be 'sync aware', i.e., Bookle should be able to deal with changes on different systems. I focus on a single sync or cloud platform should not be necessary, there is not only Dropbox out there.
Can you give some more detail about what you're looking for here? We'd like to do something that would enable syncing of books across multiple Macs via a cloud-based service, but it sounds like you're thinking about something different. What capability are you hoping to achieve with such a feature? I ask, because perhaps we need to look at it from that perspective rather than from the rubric of cloud storage.