Hello all,
To reiterate what starson17 said, when you are using dropbox to share calibre libraries between machines, be aware that you should not have calibre open on more than one of them at a time. If open more than once, it is possible to lose changes or worse.
The problem arises because each version of calibre thinks it owns the metadata database and feels free to change it whenever it wants, sometimes when the user didn't make any changes. Also, the calibre viewers store bookmarks in the books, causing them to change.
When a file changes, dropbox attempts to copy that file to all the other 'dropboxes'. If it discovers that a file has been changed in another dropbox, then one of the copies wins (not clear which) and the other is renamed something like
foo (XXX's conflicted copy as of ...)
If the file is metadata.db, then your library and database are probably out of sync, and changes have possibly been lost.
You can tell if something has gone wrong by looking for files with 'conflicted' in the name. If you see them, then some cleanup is necessary.
Because I am a bit paranoid about simultaneous changes, I keep my one-true-library on my main machine outside of dropbox. From time to time I use a replicator (smart copy program) to duplicate the library into my dropbox, which then syncs to the other clients (two laptops and another desktop). Changes made by other dropbox clients are ignored, unless I choose to replicate them back into my one-true-copy.