Gary Paul Nabhan’s book “WHERE OUR FOOD COMES FROM: Retracing Nikolay Vavilov’s Quest to End Famine” (Island Press, 2009) is a weighty tome, freighted with implications. But as befits its subject it is also portable and travels well, a deft exploration of two trips around the world, that of the author following in the footsteps of a long-gone mentor he never met, the Russian pioneer botanist and geneticist Nikolay Vavilov (1887-1943).