I think the hands down winner for Most Influencial Book is Medical Nemesis: The Expropriation of Health by Ivan Illich simply because it humbles you. In an atmosphere where doctors are becoming sickening self-congragulatory about the increasing use of technology in medicine which enables us to intervene and control bodily phenomena like never before, Illich reminds us that medicine, just like everything else has is limits beyond which is cannot hope to do good. Here is a sample.
Increasing and irreparable damage accompanies present industrial expansion in all sectors. In medicine the damage appears as iatrogenesis. Iatrogenesis is clinical when sickness and death result from medical care; it is social when health policies reinforce an industrial organisation which generates ill health; it is structural when medically sponsored behaviour and delusions restrict the vital autonomy of people by undermining their competence in growing up, caring for each other and aging or when medical intervention disables personal responses to pain, disability, impairment, anguish and death.