Color of Life: In by Will McPhail
I think I experienced a bit of synesthesia while reading In by Will McPhail.
The book is largely done in sketch-like black and white, the characters little more than outlines on a white background, except for moments when Nick, the main character, experiences real human connection. As soon as he makes that connection, the pages burst into fully painted, dynamic scenes, and I oftentimes felt like I could hear the sounds of crashing waves or the swell of some imaginary film score in my head. It made the whole thing quite extraordinary.