John Gottman approaches the study of relationships with a scientific angle. He collects and analyzes tons of data from each of the couples he works with. Things like your heart rate and sweat during an argument, or your eye movements when you’re with your significant other. Years of doing this has helped him develop heuristics which he can now use to, within 15 minutes of meeting a couple and with 90% accuracy, determine whether the couple will stay together or not.
The book is about “7 Principles” that he has distilled from that research that can help your relationship work.
I found the principles applicable to any relationship (not just to romantic ones). A lot of them are just very good techniques to make others feel cared for and appreciated.
On the other hand, my biggest criticism would be that a “marriage that works”, according to the book, in its most simplistic terms, is one that lasts; and I just can’t agree with that definition. There’s no point in making bad things last, which I feel that sometimes happens to marriages because we’ve been taught that long marriages are the equivalent of success. I think a good marriage is one that helps both people grow.