Preferably a banned book, as it is Banned Books Week. I don't know which books I have on my shelf and which I don't so instead of following Sarah's example I'll examine the lists of most challenged/banned books from 2001-2008. Of those I've read:
Bridge to Terabithia
Julie of the Wolves
His Dark Materials series
Harry Potter series
The Adventures of Huckleberry Finn
Of Mice and Men
I Know Why the Caged Bird Sings
The Color Purple
I've put these in approximate age of readers. Captain Underpants is a beginning chapter book, I think, or read to your K-1st grader. I saw it was challenged for being "unsuited for age group" - well, I don't know about the people who challenged it but Mom and Nettie had the best time ever reading it together. It's the only one on the list I haven't read all of (OK, also haven't read Amber Spyglass, yet) but I think from the bits I have read/heard that it is perfect for the age group. Potty humor and burping is right up an average 6-9 yr old's alley, parent's might not always appreciate it but their kids love it. Mom and Nettie laughed the whole time.
The next three are (my impression) for late-elementary schoolers. I read Bridge to Terabithia and Julie of the Wolves in 6th grade, both for discussion/book reports. I'll admit that 6th grade was a while ago, so my memory may not be perfect but violence? I don't remember any particular violence in either. In BtT the kids have a fantasy world with battles against imaginary foes - you can't have kings and queens without dragons - but, in my mind, that's different than 'real' violence. It wasn't gratuitous or glorified. In JotW, Julie's village hunts, the wolf pack she joins hunts, but I can't for the life of me come up with any other potential violence to complain about. Maybe I've forgotten a bunch in the intervening 20 years? IDK, IDU either.
His Dark Materials is somewhat anti-church (in my reading really more anti-establishment or anti-dogma than anti-Christianity despite, perhaps, the author's intent) and this would make it a target of many book banners. Don't want the kiddos asking too many questions. Yeah, there's some violence but not gory and how do you have epic battles with out some violence? How many kids are going to read a trilogy that doesn't involve a major world changing conflict? And how many of those world changing conflicts are going to not involve a war? Yeah, you can do it, but the central conflict of the story needs to grab kids - war grabs kids (and adults).
It seems to me that the Harry Potter series starts as a mid- to late-elementary (3-5th grade) reading and ends as intermediate school reading (7-8th). I suspect a lot of the challenges to Harry Potter are because of the magic - inherently anti-church, if you're afraid of questions. My sister has a friend who isn't allowed to read the books or watch the movies because of this (but can watch Mary Poppins, go figure.). Yeah, there's violence. Violence with consequences and not without reason (at least for the good guys). Again the good guys are fighting evil, and Harry, et al. (due to an accident of birth) get sweep up in it. It's hard to fight evil without shedding some blood, Harry, Dumbledore or any of the other of our heroes, didn't started it but someone has to end it. BTW, sometime this happens in real life, not so much an 11 yr old kid being swept up into and being central to the resolution of the war, but bad things happen and sometimes young men and women have to go off to war - many not more than a year or two older than Harry, Hermione, and Ron in HPatDH - and kill or be killed.
I think kids can handle a little more than we tend to give them credit for. Or more than the idiots who ban books think they can. Not only can they handle people in a book fighting battles or hunting their food - they can tell the difference between reality and fantasy.
*post title is poorly purloined from RIF-Man lyrics