This assumes that the font instance being searched for is an override:
- Click in some text in one of the book files.
Edit > Find/Change
[ Character Format ...]
This pops up a Character Format dialog. - Assuming you are on Windows:
[Ctrl]+[Shift]+[F8] to set everything to As-Is/blank.
In Family:, select the font being sought. [Set] - Back in the Find dialog...
Look in: <*> Book
[Find]
If the font instance is part of a Paragraph Format or Character Format, you can search by Para Tag name or Char Tag name (which is yet another reason why overrides are a problem).
Or you can save out all the book files as MIF and search with a plaintext editor, but that requires a nominal knowledge of MIF.
I might add that steps 1-3 above are covered in FM7.1 Help (and presumably the printed manual back then), but are absent for FM9, and I'm guessing that they never returned in later versions either.