I'd like to re-read I, Robot. Would be cool to see some Kim Stanley Robinson on the list.
Oh yeah, it's gold if you like the harder side of sci-fi. Hardness in science fiction, like hardness in reality, is a thing of degrees (as I'm sure you are probably aware). If diamond hard is, say, a technical manual for a warp-drive or some currently impossible thing, and the first episode of Star Trek the series is the hardness of old growth redwood, then the Mars series is hovering in the gold hardness region. It's malleable and easy to dent if you must, but it's heavy and will crush your skull a bit if you hit yourself in the head with it all at once. I did Red Mars, took a break for a couple of books, then Green Mars, etc. Hope you check it out, I'd love to discuss here!