Blue Cats and Chartreuse Kittens: How Synesthetes Color Their Worlds… by Patricia Lynne Duffy
This is an interesting and simple to understand book on the phenomenon of synesthesia. Iimagine seeing numbers as colors or words as shapes. What would music be like if you not only heard it, but saw it in your minds eye?
This book also explains that some synesthetes "see" time in a different way.