There is, but it's set something like 15-17 years in the future so it's not technically part of the love story between Maxon and America. I believe The Heir is about the next generation.
The first and third books are really good. The second, not so much, but there are a few plot points in it so unfortunately you can't skip it. I'm on the waiting list at the library for the fourth. There are also four novellas... I have two of them so will be reading those next.