This program can create a subset of a font. You can choose to either input some text (or paste a whole book) and determine the unique characters. That should go quite fast (for example, HP4 takes about 1 sec on my laptop). You can also choose some sets of characters.
You can also just drop an ePUB on the application to shrink all the fonts in the ePUB according to the used glyphs.
It only accepts TTF.
It should run on x86, probably XP and up.
Latest version can be downloaded from my site
Version 2.11 is located here