an excellent book indirectly about synaestheisa is A R Luria’s “The mind of a mnemonist: A little book about a vast memory” (amazon link )
the subject ‘S’ had a memory that was for all practical purposes inexhaustible. part of the reason for this was because he was very highly synaethetic, colours had texture, smell & emotional associations. numbers had tastes and tones, etc.. his whole world was a rich mass of interacting sensory experience.
it made even the most mundane things extremely vivid and helped him remember practically everything.. not that it helped him, he was an unassuming, unremarkable and unhappy man 🙁