can't add any fonts to font book

Usually an indication that Font Book's database and/or collections are damaged. You can reset it by restarting the Mac in Safe Mode (restart and immediately hold down the shift key), then restart normally. This will clear Font Book's database, font collections and all cache data from your user account.

Be aware that all fonts will be active regardless of their state beforehand, and all collections you've created will be gone. Unfortunately, there isn't another way to fix this type of problem.

Thanks so much, Kurt. I had this problem too and tried deleting the plist etc to no avail. My font book now accepts new fonts!

cheers.

Font book does not add fonts regardless of restarting with shift held down - safe start. Font book still contains user fonts and other lists.

Any other solutions would be greatly appreciated.

Font book does not add fonts regardless of restarting with shift held down - safe start.

That is correct. While in Safe Mode, you cannot add any fonts. The OS only allows certain necessary fonts to load.

If you mean you've done a Safe Mode boot and still cannot add fonts after restarting normally, then there are multiple possible problems:

1) The fonts are so badly damaged, they can't be used.

2) They are Type 1 PostScript fonts which are missing the suitcase of screen fonts. Printer outline fonts cannot be used alone.

3) The OS needs to be reinstalled.