I can't get through Nation or I Shall Wear Midnight. Not because they're bad, but because I get so terribly depressed and sad and they feel so final.I have told you that I love you Cyan, right?
Definitely read the next two (although Wintersmith is the worst of the 4). I Shall Wear Midnight is up there with Wee Free Men in terms of quality and is really dark by his YA standards. It reminds me a lot of Night Watch in terms of tone. I should note The Wee Free Men makes me tear up a bit towards the end every time I read it. Perfect book to read if you have a deep fondness for a grandparent that is no longer with you.
Also please say you have read Nation. If not read Nation. IMO the best book he has ever written.
The Tiffany Aching books should be required reading for middle school.