I use the excellent android app FolderSync. It can very efficiently synchronize two folders. I am sure that there are many similar folder sync apps for Windows 8.
I save the ebooks I want to have on my tablet to a folder on my NAS that I have shared over wifi. I connect to the folder as a device, from calibre, and update, add and remove books from the calibre gui. You can see what books are already in the folder and what books are not.
On my tablet I have FolderSync set up to every hour automatically sync a folder on the tablet to that shared folder, but only if charging. Or when manually requested. Works great and is very fast. There are many other similar apps. I assume also for Windows 8.
You can set up many folder pairs the same way in FolderSync. Or you can set up some cloud service to sync a folder on your computer to a folder on your tablet. See my signature for a suggestion.
I don't use that method, because my Internet connection is too slow. Much faster to sync locally over wifi.