title_body: What happens if I have used 2 different fonts in mobi/epub and user changes the font in the devise? What happens if I have used 2 different fonts in my content for mobi/epub and user changes the font in the devise,like in kindle?
  does both the fonts change? if yes, is there any way to protect the fonts from changing(apart from  converting to images)?
upvoted_answer: There isn't much you can do, if the device allows to change the fonts and the user is willing to use that option, he will do it.
  
  If you use different fonts not just for aestethics, but with a functional purpose (i.e. to display code with a fixed width font, or to differentiate quotes from default paragraphs, and so on), your best option, IMHO, is to insert a disclaimer where you explain this to the end user.