Gary Chapman has long been a favorite - he's a smart guy and an excellent communicator.
This book compares stages in marriage to seasons, and gives strategies to move from tough places into 'seasons' of health - and additionally, gives you the tools and tips you need to stay there. There is a "Marital Seasons Profile" you can take, Q&A's with lots of real-life stories, and an in-depth study guide. He provides a great deal of both theory as well as practical information, presented in an easily understood way.
It is a pleasure to recommend this book to anyone who is looking to strengthen their marriage, make some changes, learn new skills, become a better communicator....the list goes on and on. You will enjoy it and learn a lot, too!