Thank you for your interest in FBSync.
Assuming that you have FBSync set up on both devices with the same account, and set up to sync your files via Dropbox, and you are using the same dropbox account on both, then everytime you sync on a device it will upload/download any missing files on the device/dropbox automatically. It downloads them to the same location it uploaded from, so if you stored them on your tablet in
Then they should appear on your phone in the same directory. If your phone doesn't have that directory (for instance, you don't have an sdcard inserted, or /sdcard/ isn't valid for some reason), they won't download.
Another scenario that would fail is if you stored your books on internal memory on your tablet, but your phone didn't have internal memory, or the internal memory wasn't mapped to the same directories.
Without knowing the specifics of your devices and their directory structures, I'm afraid I can't give any more specific advice.