I took a bite, finding it as sweet and hot as any I'd ever had, and was overcome with such a surge of homesickness that I turned away to keep my control. I walked along, munching the yam, just as suddenly overcome by an intense feeling of freedom--simply because I was eating while walking along the street.Ralph Ellison, Invisible Man
3009 students, here are some useful links to learn about yams. All are interesting; the first, third, fourth, and fifth have the most helpful background for the novel. Thanks to Danny and Lara for these links.
"Home Cooking: Sweet Potatoes or Yams" [from about.com]
"How have yams changed medicine?" [from killerplants.com]
"Supercrop: the yam bean" [from Natural History via zinkle.com]
"What are yams?" [from HungryMonster.com]
"The world's healthiest foods: Yam" [from whfoods.com]