I use dropbox for syncing. I point my calibre library to the dropbox folder, and in my ipad I open the dropbox app and "share" the book I want to read to the Books app.
The calibre manual warns against using dropbox on the main library folder, users have lost their complete libraray doing that. Might want to at least pause dropbox while running calibre.
I have using calibre with dropbox for 10+ years now. The warning was given out mostly for the internet speeds of the time where the time for your edits to be synced took very long. I had a problem only once in 10 years even then because of dropbox revert file ability was easily able to revert.
The only caveat is I do not use calibre server so I usually start calibre do what I want and exit. If you use the server and keep calibre on all the time then Dropbox might not work as well for you though just restarting calibre after any library edits should be enough.
