It is the manner of the person who makes the correction that is usually resented. Your friend is certainly entitled to as much consideration as you show your acquaintances. Too many of us feel that we can take liberties with our friends that we would not dare to take with strangers. Handle your friend's book as carefully as you would the book of a new acquaintance, more carefully than you would handle your own. Do not feel that because it is with your friend that you have an engagement that you can be half an hour late.
Real friendship is founded on courtesy, kindness, and understanding.