Using the adding fonts and referencing them in the CSS method is tedious but it does work, although it is kluge. The problem people sometimes encounter is when the original CSS calls a particular font or font type from a tag other than the BODY tag or P tag. If the technique doesn't work, examine the original CSS to see if font attributes occur in other tags and change them accordingly.

Keep in mind that many publishers don't use clean HTML to publish their books as ebooks. They often convert print version texts and depending on the software they use they might wind up with some terrible looking and convoluted HTML. As an example, convert a *.doc or *.rtf file to HTML using Microsoft Office and then take a look at the ridiculously complicated HTML code that is generated.

Like I said this is tedious and kluge. I do wish Sony and Amazon (and other manufacturers) would add a easy to use font changing method similar to what some of the apps available for iPhone, Android, and Windows use.