I read I am Spock. I actually have a signed copy somewhere from when he was on his book tour.

IIRC, he wrote I am Not Spock as a sort of joke, where Leonard Nimoy would have conversations with the fictional character Spock in the book. It was meant to be funny and lighthearted. Apparently it came out when there was some Star Trek kerfuffle (between the TV show and the first movie, I think), and the fans just saw the title and thought that Leonard Nimoy didn't like Spock, and were really angry at him.

So he wrote I am Spock to set the record straight. It's also lighthearted, and an easy read.