While researching for my last project at school, I read a lot about synesthesia. The easiest way to describe synesthesia is a fusion of the sense. Often, when someone hears a certain sound, he or she will simultaneously see red swirls. Sometimes, as in the case of my Aunt Scarlett, certain numbers will be in certain colors (2 3 7).
I'm recommending a couple of books on synesthesia:
Born On A Blue Day is the first one. This was written by Daniel Tammet, a man who has Aspergers (a mild, highly functioning form of Autism) and strong synesthesia. The unusual thing about this book is that people with Autism are rarely able to express what's going on in their own heads and bodies because they can't communicate well in the "normal" world.
The second is The Man Who Tasted Shapes. This was written by Richard E. Cytowic, a neuroscientist who is leading authority on synesthesia. He covers the biological aspects of synesthesia in a way that's accessible, and he frames it all in a case study of a friend of his who, when he was cooking dinner, said "I need to add more points to this chicken".
Now go read. :)