The one I think that influenced me the most was Raving Fans: A Revolutionary Approach to Customer Service by Ken Blanchard. It's somewhat related to what I say in my book [Hug Your Customers].

When women (of any age) hug me, they always want to put their arms above my own -- their arms are basically around my neck during the hug. Picture me bent in half hugging my 4′11″ mother-in-law.