"Start with what you know, and what you can do well. Work out what your talents, skills and interests are first; then you’ll be able to distinguish the good ideas from the bad, and make the good opportunities lucrative. You’ll also find developing — and maintaining — your product or service a whole lot easier if it lies within your area of expertise and passion."

http://webworkerdaily.com/2010/06/15/dont-write-that-e-book/