I went down a short bunny trail trying to figure it out.

https://en.wikipedia.org/wiki/What_Is_It_Like_to_Be_a_Bat%3F

> Nagel asserts that "an organism has conscious mental states if and only if there is something that it is like to be that organism—something it is like for the organism."

I want to answer this question as if you asked which book had the biggest impact on me, that I remember with the most fondness, the one I wish I could forget so I could experience again from zero.

Cryptonomicon by Neal Stephenson. Long and sometimes tedious, as his books can be, but the characters are so memorable. Bobby Shaftoe is one of my all-time favorites. The book takes place on two timelines, involving two generations of characters, that have interesting parallels. The audio book narrator does a great job, if you're into audiobooks.

Runner-up would be Warbreaker by Brandon Sanderson. His best, IMO.

I still use Stephenson’s definition of “failing an intelligence test” from those books. I’d heard the term before, but the example he uses is concise.