Wonder, by R.J. Palacio, is really a kids book. I guess to be more specific, it's a book for pre-adolescents. It's has a fantastic reputation and is very well liked by nearly all that have read it; I too am happy to have given it a spin.
After reading it, you can see why this book appeals so much to the pre-teen crowd. More than anything, it's written on their level. I mean that both in the reading level of the book being very simplistic, but more so the themes of the book connecting to (pre and) pubescent boys and girls: schools, cliques, peer groups, mean kids, being different, trying to fit in, etc. At the heart of the book is a boy named Auggie, who as you might have guessed from the cover, looks different. No real description is ever clearly laid out but you understand that Auggie's physical appearance is the crux of many interactions in the book.
The story is told in a pretty fun way. About 8 different chapters are written from the point of view of a few different people in the novel. Of course, most is Auggie himself, but his friends, sister, and others take on small parts to better give you a more clear perspective on everyone's first person view of about a year's worth of events.
Overall, it's a very nice book. It's not going to blow your mind, at least not mine as an adult, but it's a great story to read and has some wonderful themes and take-aways. What I actually appreciated most about reading it was just how difficult the middle school years can be. Both working with this age population of students, and thinking back to my own time during those years, it was so nice to be reminded in such a vivid way about how critical peer relations are during this time period and how much emphasis is put on being 'normal'. I can see why this book is such a hit with kids this age. So many of the themes and ideas are so easy to identify with and so many of the characters play out in similar ways in middle schools all across the country.
Thankfully, a book like Wonder comes along and both kids and adults can enjoy some of the great lessons learned from this piece of literature. I do urge you to pick it up to remind yourself why those lessons are important no matter what your age.