Dr. Paul Farmer is so busy toiling as a modern-day saint that he forgets to eat. He wouldn’t be able to buy food anyway because he has a habit of signing his paychecks over to AIDS patients. Farmer forgets to sleep, too (he averages about two hours a night) — how else would he find time to shuttle between his hospital in Boston and the relief clinic he founded in rural Haiti? Farmer isn’t just a saint; he’s a miracle. But the irony is that no matter how many patients he squeezes into his day, he still senses that he’s a failure who’s bailing out a sinking boat with nothing more than a teaspoon. Set amid the political turmoil and crippling famine of the past decade in Haiti, ”Mountains” is typical Tracy Kidder — which is to say, typically great. Like his previous books on technology (”The Soul of a New Machine”), the elderly (”Old Friends”), and American education (”Among Schoolchildren”), the Pulitzer-winning author turns the small details of daily life into a sort of grand, universal poetry.