I just finished reading 'Stop Walking on Eggshells: Taking Your Life Back When Someone You Care About Has Borderline Personality Disorder by Paul Mason MS, Randi Kreger. Not sure what I think of it. The first part felt like they were writing about my H, but then later in the book, to me anyway, it sounded too much like you are supposed to keep living with the BPD person. Sure, if people choose to, its good to know how to best deal with them. But then it made me feel guilty for wanting to leave.
I read Stop Walking on Eggshells a couple years ago already so I don't remember it all, but my recollection was that a lot of the book was directed towards the idea of that if you have chosen to stay in a relationship with someone with BPD, then this is what you need to know and these are the steps you can do to minimize your partner's negative reactions and to maximize your own mental health. But that is NOT to say the author is trying to tell you that you should stay in such a marriage. It is just a guide to help you in case you DO decide to stay.
So I did just quick-flip through it and I think you'll find Chapter 12 discusses the questions you should ask yourself to help you decide whether to stay or leave. The key issue is whether you can be happy in such a relationship and whether you can make the kinds of changes you need to make to be healthy. You cannot get your partner to make changes though you can set your own boundaries which will result in certain changes. So you would really have to decide whether implementing the strategies described in the book are things you can do and then once you have tried them, is the net result something you can live with despite the fact that a BPD person will always have BPD issues.
But I really didn't have the sense that the book was intended to make you feel like you should be able to be happy or feel fulfilled doing all of that. That is something that only you can know for sure. And nothing can force you to be happy about something or comfortable about something that you honestly just plain are not!!!
Also, although there are some things in common with a BPD and an NPD, they really are quite different, too. I had the impression that you were thinking it was more an issue of NPD with your H. And I get the impression that is a bit tougher to deal with even than BPD. (And for that you might want to read The Wizard of Oz and Other Narcissists: Coping with the One-Way Relationship in Work, Love, and Family.)
I haven't read the Splitting book, though and I don't remember anyone reviewing it here. So let us know what you think once you've finished it!