I've been trying to figure out what I enjoy so much about Crowley and Aziraphale - and I think it's about how they don't change for each other. They are very honest about what they are and what they can't be or do. Crowley's a demon, yes, but he more sauntered away from the path than fell, right? And he sure as heck isn't on board with killing babies, dammit. Azi is a fairly flawed angel, but a sweet and funny spirit. And somehow over a few thousand years they learn to enjoy each other's company and become more important to each other than almost anything else.
Also love that they adore earth - good wine, nice restaurants, park benches under trees and people to watch.
I take it the TV show's more a love story than the book (author's words, not mine) - so I'll be interested to see how the original strikes me. Time for a visit to the kindle store.