RV,
Are you looking at this from the standpoint of interest, or are you actually wanting to design heat exchangers?
If you are intending to do design, then I would recommend registering for the appropriate course(s) at your local University. Heat transfer can be quite complex and requires a strong background in mathematics, fluid mechanics, physical properties...as well as understanding the principles of conduction, convection, radiation, etc.
If you are looking for a text to "broaden" your knowledge base, I have an old copy of "Heat Transfer" by J.P. Holman that I like. "Process Heat Transfer" by Kern is a bit outdated...but still proves to be a good reference.
Good luck!
Bob