This is a "companion book" (NOT a sequel) to Wonder. It contains chapters about Julian (who was a real jerk in Wonder; also one of Auggie's "welcome buddies" and classmates at Beecher Prep), Christopher (who was Auggie's first friend when they were babies together, but who moved away), and Charlotte (one of Auggie's "welcome buddies" and classmates at Beecher Prep).
This book is not as good as Wonder. How could it be? Wonder is one of the best books I have ever read.
Having said that, this book is interesting and quite good in its own right. It would be essential to read Wonder first. This book does not really add anything to the story of Wonder, but the stories of these other young people are interesting in their own ways. I am still not a huge Julian fan, but it was interesting to read about his perspective on the events of the previous book, and what he ended up learning from it.