Oh man, this is such a pretentious answer, but Philosophical Investigations by Wittgenstein. Seriously.
The basic idea of the book that you cannot think straight until you understand the hard limits of thought, and respect it can never be more than a supplement to performing useful actions in the world, is a lesson I've never unlearned.
The basic idea of the book that you cannot think straight until you understand the hard limits of thought, and respect it can never be more than a supplement to performing useful actions in the world, is a lesson I've never unlearned.