"I never read a book before previewing it; it prejudices a man so"

This quote by Sydney Smith is a tip to not evaluate a book by its cover. He is suggesting that one should not form a viewpoint of a book before really reading it. He believes that forming a viewpoint of a book before reading it can be harmful to one's understanding of the book. He is warning against forming an opinion of a book based on its title, cover, or reviews. He thinks that forming an opinion of a book prior to reading it can result in a prejudiced opinion of the book. He is recommending that one need to read the book before forming a viewpoint of it. This quote is a tip to not let presumptions affect one's opinion of a book. It is essential to check out a book before forming a viewpoint of it in order to get an objective understanding of the book.