That's exactly the stuff I was thinking of too when I wrote that. I've always enjoyed concept art books, my first exposure being the old Star Wars concept art books, I like seeing the design evolution.
In the Art of MGS book, even the discarded designs are awesome! I like all the various ideas for Old Snake and Raiden. Off the top of my head, I also remember a very badass concept sketch of Meryl, with pretty muscled arms and a bandanna.
A real shame they did not release an Art of MGS3, I'm interested in what the unused designs for that are like.
The closest you'll get a Art of MGS3 is that HD book. About 75% of that stuff I've never seen before.
The original concepts of REX looked like ED-209 from Robocop lol.