"The Road" has already been mentioned as a piece of fiction that focuses almost exclusively on food and starvation. Not sure it's explicitly about nuclear war, however. It's also way too hopeless for me. I can't say I enjoyed reading it.
A better -- and recent favorite of mine -- sci-fi book about scarce resources and food is "The Windup Girl" by Paolo Bacigalupi. It's not about nuclear war either, but not only is the actual story very engaging, it focuses on how energy is generated and stored, what people eat, and how food is delivered in a post-oil world. Bacigalupi calls this crisis "the Contraction". Warning: more than a few digs at Monsanto as well, not so much from the "transgenics are bad" angle but from the "copyrights and patents on food oppress people". Strongly recommended!