One rule: nothing built on scripts, closing tricks or manufactured urgency. Those techniques can win a single transaction, and small business runs on repeat custom and referral, so winning that way is expensive.
The books that made it treat a sales conversation as a diagnosis — working out quickly whether you can genuinely help, and making it easy for the other person to decide either way. Owners who hate selling usually find this version recognisable, because it is close to what they already do when explaining their work.
The other thing we looked for was honesty about volume. Sales is partly a numbers game, and a book that pretends the right technique removes the need to have enough conversations is selling comfort.