I don't know if it was "heavily marked", but yes it was among the titles they found. They also found a copy of "The Basics of Rifle Shooting" from the NRA, and Mark Twain's "Life on the Mississippi. He was a voracious reader.
If you read his manifesto, his references to the environment and environmental activism are comparatively limited, and are generally in the context of how a degraded environment effects the quality of human life, and not how human life degrades the environment, which I would suggest, contrasts sharply with Gore's thesis. Again, I'm not trying to be a Unabomber apologist, but I think accuracy is in order when discussing him in the context of today's events at the Discovery Channel.