Scott Allan at the Athority Pub blog wrote a good article about the ten elements for nonfiction books.

I have to strongly agree with his last point, ‘Write in a ‘friendly voice’ that readers can relate to.‘  I’m amazed at how many nonfiction books have a voice of a professor talking down to students.  The better way of a friendly voice that is educating, is far better and will make a reader want to keep reading… and buying more of the authors books.