The narcissist's second book primarily focused on leaving romantic domestic partnerships with narcissists and so it did not apply to me. I chose not to purchase any more of his books and as a last-ditch effort decided to visit the Out of the Fog website for some final insight(s). I picked up some valuable info there that I will refer to in the future.
I sent my Dad an email similar to the one that I described; however, it was a bit more tactful. I decided to cut to the chase and tell him that as our relationship stands, I have put forth my best efforts and that he should spend the rest of any time that my stepmom has remaining with her. I told him that should the unfortunate circumstance arise when she passes, he is free to contact me if he chooses to do so (at which time I will initiate one of the communication methods I read about at Out of the Fog). I think that gets my point across far more effectively than being angry and that my decision to give myself some space will in turn give him something to think about.
I have an appointment with my new therapist late this afternoon where the primary focus will now be on taking better care of myself. It's a brand new day.