I remember reading that book. His first book was called "I am not Spock" and was talking about all the stuff that's Leonard Nimoy alone, but this one was about how he'd internalized a voice from his character who would have the Vulcan point of view.

One of the most interesting parts of the book was where Spock pointed out there's no need to fear death because it's non-existence, and you won't be there to care. Considering I read this when I was still in grade school, it was a pretty big revelation.