Synaesthesia is very personal. I experience tones, keys, chords also
as colours but I'll never be a Messiaen I'm afraid. I have the same
thing with numbers and the days of the week - so maybe I should keep
a diary ;) The most interesting book I ever read on the subject in its
extreme and pathological form is by the Russian neurologist (and Oliver
Sachs-predecessor and inspiration) Luria. It's called *The Mind of a
Menomist*. I am off topic here of course but the book *is* something else.
Philip