Hey y’all!
What are some non-fiction books you’d suggest about marriage? Can be anything from “how do you know you’re ready” to “setting boundaries” to any marital topic at all, really - even nonfiction that analyzes how the institution developed in the Western world, etc. All grist to the mill, so to speak. Things you’ve personally read and would swear by preferred. 
Or, if you’ve got a book you’d swear AT and actively discourage people from reading, feel free to share that too.
More of a memoir or personal reflection than an advice/how-to book, but how about Madeleine L’Engle’s Two-Part Invention: The Story of a Marriage.
I’ve read and enjoyed another book by this author, though not this one in particular: Marriage, A History: How Love Conquered Marriage. by Stephanie Coontz.
Gary Chapman’s The Five Love Languages. He’s Christian, so if that isn’t your thing, there are parts that might make you roll your eyes. However, he’s not preachy and those parts are small.
It’s a great tool for seeing the good parts of your marriage and working together to recognize what you are truly doing for one another. Gratitude and feeling loved are two very important aspects of a happy marriage. This book helps with both.
These are either Scriptural or Christian based so if that doesn’t apply to you then you probably will not like these but I have found them invaluable…
Love and Respect: The Love She Most Desires; The Respect He Desperately Needs by Emerson Eggerichs
Real Marriage: The Truth About Sex, Friendship and Life Together by Mark and Grace Driscoll
This Momentary Marriage: A Parable of Permanence by John Piper