It's title is Wednesday Is Indigo Blue: Discovering the Brain of Synesthesia. I haven't read it yet. Our state library service is so hopeless that they might never stock the book, or at least take many months to provide a copy, so I might have to buy it.
And I'd like to point out that Wednesday is certainly not indigo blue coloured, at least not in my universe!
Link to interview with Richard Cytowic in the April 2009 edition of Scientific American Mind:
Link to a review of the book (by a synaesthete) at New Scientist:
Link to book at Amazon.com: