i don't have font book on mojave. where can I find it to download

Are you sure you don't have it -- it's supposed to be located in the /Applications folder -- did you remove it or move it somewhere? How did you do it? I would really suggest not removing stuff from your /Applications folder, or other locations, that Apple put there for exactly the problem you're having now. If you did remove it, perhaps it's in your Trash folder?

Anyway, you can't download it separately, so you might have to install one of the Mojave combo updates from Apple's downloads webpage at https://support.apple.com/downloads and search for the correct one -- most likely the 10.14.6 combo version, but if you're not on 10.14.6, you might want to pick whatever version of Combo update you are on instead.

Good luck...

tclements wrote:

I need to add additional fonts on my MacBook Pro.

The normal way to do that is just double click on the font file and then click on the "install" button. You don't need to do anything with Font Book.

Font Book is incorporated in macOS and cannot be obtained separately.

To reinstall macOS please read Reinstall macOS - Apple Support.