I find that people tend towards books that suit the reader. So how well this particular book might teach you may have more to do with what you're looking for, or what would suit your situation.
With that in mind, Opening Up is primarily oriented towards skills that established formerly monogamous couples could use to open up towards non-monogamous relationships. If that suits your situation, or mindset, then it'll be a good starting point.
On the other hand, if you're starting from a more single-ish, or less exclusive arrangements to begin with, then you might get better value from something like the Ethical Slut which doesn't predicate on the basis of being part of a couple.