On a completely different topic: After watching my first marriage degenerate and my wife asked me to move out, I was fairly well mystified about what was going on. We'd read lots of books about marriage and communication, planned for it to be tough, intended to see it through... so what happened?
"The Seven Principles for Making Marriage Work" by John Gottman was a book that I read after we'd separated that first started to make sense of the dynamics of my first marriage, and what caused it to spiral out of control. Unfortunately by that point it was basically too late (as the book predicted, actually); but it certainly helped a lot in my second marriage, and has helped make sense of the dynamics of a lot of other relationships as well. Definitely recommended reading.
What Makes Love Last is really a good one too. Helped me to really understand that love is about trust and what that meant. John Gottman is a treasure.
The key was that it's not just one thing, but interlocking self-reinforcing things. (There's another book of his, "The Mathematics of Marriage", targeted at academics, where he actually models a bunch of things as mathematical equations.)
The core idea is what he calls "sentiment override" -- positive sentiment override, and negative sentiment override. Basically, your experience of someone is modified by your attitude towards them. When you have a positive attitude towards someone, negative things get diminished, neutral things become mildly positive, and positive things become very positive. Whereas, when you have a negative attitude towards someone, then positive things get diminished, neutral things become negative, and negative things become massively negative.
And it should be clear that these are self-reinforcing: If your relationship is in a "positive sentiment override" state, where neutral things become mildly positive and negative things are diminished, then it's easy to stay in that state. Whereas, if your relationship is in a "negative sentiment override state", where positive things are diminished and negative things are magnified, it's really tough to dig yourself out of.
So he's got a set of warning signs he calls "the Four Horsemen of the Apocalypse", which are indicative of problem marriages: Criticism[1], Contempt, Defensiveness, and Stonewalling. He said that when couple exhibits all four of those when discussing a contentious topic, there's a 95% chance they'll be divorced in 5 years. (And indeed, by the time I'd read the book, all of those were deeply embedded in our communications; about best we could hope for from any conversation was to avoid having it turn bad.)
[1] He has a particular definition of "criticism" which is similar to but not a 1-1 match with the way it's commonly used.
And he's got a bunch of things to do to strengthen your "positive sentiment", to keep you on the positive side rather than the negative side.
And all of this comes from studying loads of marriages -- both healthy and not -- and seeing what made the difference.
Hopefully that's whet your appetite enough to go take a look at the book yourself. :-)
'Mating in Captivity' by Esther Perel is also quite good in terms of realizing whether you can meet the Needs of your partner and what to do if you cant.
His book presents very reasonable advice but his statistics are made-up. Doesn't mean it's not a valuable book but can be a bit annoying for people who care about being accurate.
"The Seven Principles for Making Marriage Work" by John Gottman was a book that I read after we'd separated that first started to make sense of the dynamics of my first marriage, and what caused it to spiral out of control. Unfortunately by that point it was basically too late (as the book predicted, actually); but it certainly helped a lot in my second marriage, and has helped make sense of the dynamics of a lot of other relationships as well. Definitely recommended reading.