The top contenders:
Creature Collection Revised will give you the standard, setting beasties, minus the sutak. It has the asaatthi, ratmen and other creatures that'll give a slightly different look from other settings.
Then there's also the Divine and the Defeated. Personally, I think this is the best book out of the entire line, so long as you're just looking at it as a setting rather then what you can get more non-Scarred Lands use out of. It goes into the religions, both divine and titanic, for the setting. It stats out the gods, sure, but the stats are garbage and better ignored; the meat of the product is the sects, history and mythology. Stylistically, it's well-written all around.
Then there's the first Relics and Rituals. It's a solid, inspiring book that, while it has some mechanical problems, helps establish the setting through its prestige classes, spells and magic items. Even if you don't bother much with the Scarred Lands setting, this book should see some use outside of it.
Other notables would be Hollowfaust, which can be ported over into other games that aren't Scarred Lands. The Termana hardcover is also good and offers a different perspective on the setting.