What are great books to read to gain knowledge?

Depends on what you want to know. A well rounded person reads a bit of everything. Read the great classics by Austin, Hemingway, and others. Read philosophy by Plato and others. Go to museums and look at the great masters, read about them, and look at the ancient arts. You can never know too much. Pick a subject that really appeals to you and read all that you can. Mine is history and I learn something new each day.