One of the biggest mistakes authors make is thinking their newsletter is about selling. Marketing, which is what your newsletter is part of, is about building relationships. When you have good relationships with people, they’ll buy without you “selling” at all.
When you imagine that one person you’re writing your newsletter for, also remember that you’re not dragging a friend down the alley to mug them (because it’s not about money.) Instead, you’re chatting with someone who asked you to talk to them. Would you really say “I’m sure you don’t really want to talk to me, so I’ll keep this brief and infrequent”?