The Westing Game
by Ellen Raskin
I think this is one of my favorite books for children. It's a great mystery that even had me guessing. Because it was a kids' book, I thought I had it all figured out early on, but I was wrong. It's a very fun read, and it's well-written, with a wide variety of delightful characters and plot twists. The story is actually quite complex.... my daughter read it in 3rd grade (when she was 8) and loved it, but she was confused at a couple of places so I had to help her out. I would probably recommend it for those who are at least 9 and are very good readers. However, this is also a great read for teens... and their moms too. :) It's quite clever.
I don't remember anything offensive other than maybe a phrase or two such as "shut up," and a girl kicking someone in the shins.
Age recommendation: 9 - 99