Michel de Montaigne, The Complete Works
Six months ago, I got into the habit of losing my mind. No day passed without some evidence of madness: depression, compulsion, mania, panic. Nothing helped--least of all the gray city where I live. One morning while reading this book, I felt my mind click back into place and I knew I would be all right. Since then, the Essays have been, for me, a touchstone of sanity. There is something about their boundaryless curiosity, their open admission of human frailty and mess, that pulls me back every time. It's a book of ideas that never forgets about blood, sweat and semen. Every day I sit with it there is some useful treasure. Today I was grateful to be reminded, "It is not victory if it does not end the war."
Or how about: "No quality embraces us purely and universally. If it did not seem crazy to talk to oneself, there is not a day I would not be heard growling at myself, 'Confounded fool!' And yet I do not intend for that to be my definition."
I distrust Montaigne's opinions on women and God--but to be right about mankind and life on Earth is a lot. As heavy as it is, this big book is always in my bag. Spend some time with it--it will help you stay sane.