Originally Posted by omro
And how do you break inside to change it?
.ePub files are really .zip files.
If the ePub files are encrypted, the CSS may be in the clear (in ePub, files are encrypted individually) for you to edit. I'm not sure what most publishers do since I don't buy DRM ePub.
Once you have access to the CSS, modifying the font is generally very easy (look for instructions on setting font-face with ePub on these forums).