Mine was two things. The noticer by Andy Andrews and The Celestine Prophecy by James Redfield. Both books expanded my perspective about self-love and the expectations I was placing on myself. They helped me jump out of the little box in my head and let go of old identities.
The flow on effects were remission for multiple autoimmune conditions, learning how to love myself and be at ease with the life I was living... the list could go on forever really.
What was it for you?