For me, it's been "Do the Impossible" by Jason Drees - who also happens to be Brandon Turner's coach. Brandon has a foreward in the book that is really humbling and inspiring. He explains how critically important it is to do hard work that flows - there is no resistance, instead you are fully inspired and in alignment with who you are and where you are going. I highly recommend this read - by far, Jason is one of the most impactful and least discovered coaches I have found. 

Whether a business book, personal development, or a book on investing, what is the most under-rated book that has left a lasting impression of your life?