In Poor Richard's aphorism, "He that composes himself is wiser than he that composes books". What is the moral lesson that is being taught?

The translation is that a person that understands himself or herself then manages to control themselves is a lot wiser than a person who drives his ideas home to others through writing.

What he means is this: when a person can understand themselves and accept themselves, they have achieved a lot more than a person who looks outward attempting to teach others how to live.

The lesson is focus on yourself, learn how to live instead of trying to teach others how to live.