Heinlein has to be one of the most free thinking writers I have read. I find myself wanting to find the worlds he creates. I truly enjoy reading his work though he can leave you hanging with no real ending. Its difficult to say which book I like the best. Stranger in a Strange Land, Friday, or The Number of the Beast. Each I find wonderful in their own way though perhaps Friday is the one I would recommend reading first. Guess I have a soft spot for female heroes. Boy is Friday a heroine. 

Whatever book you choose read them with an open mind you will enjoy them more. Even Starship Troopers, another favorite, even if the movie was a bit gung-ho.
Books:
  • A Stranger in a Strange Land
  • Friday
  • The Number of the Beast
  • The Cat Who Walks Through Walls
  • Methuselah's Children
  • The Moon Is a Harsh Mistress
  • Starship Troopers
  • The Puppet Masters
  • Job: A Comedy of Justice
  • Have Space Suit-Will Travel
  • Rocket Ship Galileo