We are enjoying both of these graphic novels so much. Here's an interview with Nimona author, Noelle Stevenson on NPR, and also a review of the book. We saw this at Powell's and snatched it up immediately.
Roller Girl we have had for a few months and keep going back to. It's laugh-out-loud funny and takes place in Portland, with many recognizable places and references, which is really fun. There's a podcast interview with Victoria Jamieson (who is also an in-real-life roller derby skater) and a review of the book over on Geek Dad, and again, we bought ours at Powell's books.
Both these books feature girl protagonists that are real, funny, strong, and flawed. They learn from experience — sometimes. Sometimes things are just hard for them and they don't really learn. It's pretty great. All 3 of my girls love both books, (ages 8-12) Content wise, it's really a personal thing—you know your kids and what they might like and what you are comfortable with them reading. I always recommend pre-reading or at least skimming first.