First thing to say is that I never had a breakup, so I am reading this from an unexperienced viewpoint. I am not arrogant enough to assume that I will never have any breakups in the future, so I picked up this book.
Part I is a solid overview of possible causes for polyam breakups, divided in "non-polyam causes" and "polyam causes".
The prevention-part ist mostly about knowing what you want, knowing your limits and red/green flags, which is a good advice. When you are polyamorous it might be a good idea not to date someone who thinks they can convince you to become monogamous again and vice versa because that is a possible cause for a breakup, right? Thus, preventing to pick the wrong partner is good. But what if you are already in a poly/mono relationship? Of course nobody should force their beliefs on their partner. But here the author wrote, that poly/mono seldom function, maybe, just maybe, if both partners don't identify fully as poly/mono. Well, I am in a poly/mono relationship and while I have a very relationship anarchistic mindset my husband thought long about it and decided that he is completely monogamous - it works very well, because we both respect each other and have a healthy communication. No breakup in sight yet and I am very glad that after 14+ years of a beautiful monogamous relationship he gave me the opportunity to live my polyamory a few years ago. My other partner is monogamous as well, by the way. I would have a lot of things to say about how to communicate these things, these I missed here. I think Polysecure is a really good book that can help with going through conflicts and how to solve them, because usually conflict solving is a big part of avoiding breakups.
I can't say so much about the aftercare part - don't think you wasted your time in the relationship when it's over, don't forget to maintain your other relationships, that sounds like the good thing to do, yes.
The chapters were all sprinkled with breakup stories from interviews the author had with real polyam people, these were a bit repititive after a while to be honest. I know, real life examples are proving the point, but after a while I felt like I was reading a telenovela, as these were a big part of every chapter, breakup story after breakup story.
What I don't like about a lot of polyam books is that they quote evolution psychologists. Evolution psychology is debunked and it should be unnecessary to proof wether your relationship preference (or sexuality, or gender) is "natural", since it's none of everyone's business as long as it doesn't harm anyone (which it usually doesn't if it's between consenting adults). In this case it said jealousy is natural because "humans were always bad in sharing" and I have a problem with this wording because partners are no objects to "share", they are humans who decide to have more than one relationship. I don't like the thought of my two partners "sharing" me as I'm not a pizza with a limited quantity of slices, so wtf. Of course jealousy is natural, but not to be jealous is not "unnatural" either. It's a spectrum and no, not everyone who doesn't feel jealousy is arrogant and thinks they are "more primary" than everyone else and is lying to oneself. You can not feel jealousy and be perfectly insecure or even realistic about yourself, I'm the living and breathing example.
I can recommend to read this anyway because it has some good advice and I think a lot of people try to avoid the topic "breakups" because it's uncomfortable. But it is good to think about potential causes and different ways to deal with a breakup, so it's good that a book like this exists for polyamorous people!