What's Left of Me was basically about a society where every body starts out with two souls, and in most bodies, one soul dies off sometime in their early life. However, some bodies remain with two souls, like the main characters, Eva and Addie (who share a body). This is confusing to explain, but it seems really simple in the book. There is a lot of character development from basically all of the characters, too.
What's Left of Me is one of my favorite books, and I really hope Kat Zhang writes more books besides this trilogy.