To add fonts to Font Book on a Mac, follow these steps:

1. Download a font file from a site like  https://www.dafont.com/
2. Open Font Book.

3. Go to File > Add Fonts to Current User.

4. Select the font file and click Open. Alternatively, drag the font file onto the Font Book icon in the Dock, then click Install.

Once installed, the fonts should be available in most apps. If they don’t appear, try quitting and relaunching the app. Restarting your Mac may also resolve the issue.