Seconded.Fobok said:For a D20 book, though it's out of print, I'd suggest A Game of Thrones by Guardians of Order. If you've ever read George R.R. Martin, you'd know how gritty the setting is. But there is very rare magic, and when it does come up it comes up very powerfully. There's not much in the way of magic rules, because it is so rare, though.
The license for the setting has gone to Green Ronin, who will be putting out a new game (not D20) for it later this year, too. A Song of Ice and Fire RPG. Not sure what it'll be like, though.
I believe you could do a very gritty game with 4e with a few simple modifications but you may want to use something like GURPS for more 'realism'. Not sure what other systems would work for that.