Down Among the Sticks and Bones by Seanan McGuire

Before they went to Elenor West's School for Wayward Children, Jack and Jill went to the Moors.

This is the second book of the Wayward Children series. Canonically it takes place before the events of the first book. It follows Jack and Jill and shows us the world they belong to.

Getting to see why Jack and Jill act the way they do was fascinating. Seeing Jacks OCD developing and Jill's mania as well as meeting the people who helped shape them.

This is one of the few books with twins that I've read where the twins have so little to do with each other. The way they were raised was negligent at best. They weren't considered as anything except props for their parents machinations. So when they tumbled into a new world they became who they actually were, and wanted to be all along.

This was an excellent entry to a wonderful series.