The
reply I posted in your other thread about the same thing has links to the FrameXML files that define all the font styles used in the default UI, as well as a fully working font modification addon that catches everything.
If you really need to identify the specific font object used for spell names in the spellbook, just go look at the FrameXML code (again, see the links in my other post) that creates those font strings, and see which font object they inherit from. There's no way to determine this information in-game; once something (a font string or anything else) is created, there's no API to find out what it inherited from.