This has been a year of re-reading for me, so this is a timely question. I can't pick just one at the moment, but I'll split a couple of top choices via genre. (Also worth noting that in the end, my final choice would probably be the Bible.)
Fiction: The Heart Is a Lonely Hunter - Carson McCullers
Angle of Repose - Wallace Stegner
Non-fiction: In Good Company - Stanley Hauerwas
Into the Silent Land - Martin Laird