remember that a person’s name is to that person the sweetest and most important sound in any language

People love to hear their own name.

People who have read this book seem to think it is polite or pleasant to repeatedly use a person's first name in conversation.

Perhaps it was so, in the 1930s. However, I suspect that something has changed since then, namely, the popularity of this book with salespeople. Today, repeatedly using a person's name in conversation makes you sound like an overly ingratiating salesperson who's read Dale Carnegie. Don't do this; it's creepy.