Originally Posted by Barty

I think the best way to do this is to install your own font. This will work on ALL mobi books, and you don't need to run them through calibre conversion first. You can either use the hack that requires jailbreak (see developer sub forum) or the one that does not require jailbreak. Google "change kindle font without jailbreak".

The best part is you can select whatever font face and weight you like. Many fonts have a Medium or Demi variant that work well. With a font editor, you can really tweak it to get it exactly as you want.