Though not specifically about breaking up with an NPD, "Getting Past Your Breakup" helped me.
I found it helpful to learn a ton about NPD/ASPD as it helped me realize that what happened in my marriage wasn't my fault and there was nothing I could have done to achieve a different outcome.
"The Sociopath Next Door" was helpful, as were "Emotional Vampires" and "Narcissistic Lovers."
When a specific passage really spoke to me, I wrote it down (on paper in a journal, as well as saved as an e-mail draft) so I could reread as necessary to help me through this.
I know there were more books that were helpful -- I'll try to remember to check the list of books I read and repost. But I found a bunch of them by seeing other recommended books on amazon.
You can get these at your library if funds are low.
Good luck recovering -- it can be done!!!Me: BW, divorced, now fabulous and happy!
Married: 11 years, no kids
Character is destiny