Martin Fowler's Refactoring: Improving the Design of Existing Code has already been listed. But I will detail why it has impacted me.
The essence of the whole book is about structuring code so that it is simpler to read and understand by humans. It teaches me strongly that the code that I write is meant for my colleagues and successors to consume and possibly learn something good out of it. It inspires me to consciously program in a manner that leaves people praising my name, and not cursing me to damnation for all eternity.
