For me it is The Law of Attraction. It completely change my life and I was wondering what else like this did I not know before?
But also when I was 14 my dad told me to stop talking about myself all the time. It was a revelation. I thought if I tell everyone how wonderful I am then they will believe it. From that point forward I didn't talk about myself, instead focused conversation around the other person and my friendships blossomed and I became more humble and less of a self absorbed twat.