Didactic:
A term used to describe a work that teaches a specific lesson or moral or provides a model of moral or ethical behavior thinking.

Example:
"Thomas Godfrey, a self-taught mathematician  great in his way, and afterward inventor of what is now called Hadley's Quadrant. But he knew little out of his way, and was not a pleasing companion; as, like most great mathematicians i have met with, he expected universal precision...."
-Autobiography of Ben Franklin

Explanation:
This is a passage from a novel that has a didactic tone through the whole novel. The purpose of this is because Franklin wants the reader to believe and learn from his life experiences. He wants to teach the reader not only about his life and who he knew, but how to live life. This tone allows the reader to trust believe what Franklin says and shows Franklin's sophistication.