Symlink vs Hardlink and Backing up Win10

Most utilities can't make the difference between hard links and files in a Windows environment. You would need to ask your backup software manufacturer how he handles hard links. If he does not his software will blow up the size of your backup.

That could apply to your Dropbox quota as well although Dropbox might not be affected internally because they would get back the additional storage spent by you using a technique called "deduplication".

If your use case is just about finding books by clicking on symbolic links in Windows explorer using symbolic links might be sufficient.