No problem, here goes (it may not be completely accurate):
Sans_Regular.ttf: Used in the Settings & Experimental page for the explanation texts. Also for the dictionnary/note popups.
Sans_Bold.ttf: Used in the menus, in the top & bottom panel, and as the collection title when browsing a collection, and the inline text in the book & collection browsers (ie. the 'delete this item' stuff & co)
Sans_Italic.ttf: May be used alongside Sans_Regular or Sans_Bold somewhere ^^
Sans_BoldItalic.ttf: Used in the book list, for the collection names
Serif_*.ttf: Used in the reader, that's the font family your books will be rendered with.
Serif_Bold.ttf: Used in the book & collection browser for the book title & author. (NOTE: For these purposes, Serif_Regular is used instead on FW 3.x)
Mono_*.ttf: AFAICT, nowehere except in the browser, but I may be mistaken. (Might be used in the reader when you're feeding it a raw txt file or some raw html files, depending on styling).
And since v3.8.N, on FW 3.x:
CJK.ttf: Used for Chinese/Japanese/Korean scripts.
I18N.ttf: Last fallback for non-latin & non-cjk scripts.
Last edited by NiLuJe; 09-09-2011 at 10:12 PM.