I can also highly recommend Heinlein's Stranger in a Strange Land, which I finished not too long ago. I balked at it for a long time because of all the Boomer/hippie hype, and expected much of it to be drippy, free-love type stuff. But then the fey, messianic hero of the piece starts literally disintegrating S.W.A.T. teams with his mind, and suddenly it got a lot more interesting.
There were parts that dragged & others I had to push myself through, but it was worth it, and it's definitely a book that stays with you for a while afterwards.