This is so much better than The Ethical Slut (which I probably should downgrade by now) and features a wide spectrum of non-monogamous relationship models. Opening Up is more concise, cuts away all the new age nonsense (no chi or chakra!), and features a snowballed sample of ~100 people which were interviewed for this book. I know, the plural of anecdote is not data (especially for snowballed interviews), but even if the interviewees are not representative you gain a much broader perspective.

Some of the "recommendations" are a bit too much OCD for my taste (on the other hand if you're a pilot or just love to tick checkboxes you might enjoy the contract-stuff) but then those "recommendations" are just that, it's not that the author really thinks you need to adopt those if they don't fit you/the people in your relationship(s).

If you want to read a book on the topic I'd say you should go for this over The Ethical Slut, but your milage may vary.