I do love it when inspiration strikes. Perhaps relief would be closer to the actual feeling. Second books are difficult. You have to meet the promise of the first one, while subtly surpassing it...who can just deliver that? I was inspired by a fellow writer (Liesel Schwarz, whose first book, A Conspiracy of Alchemists is out in a few days, buy it, you will not be sorry) when I saw her second book. Wow. Take an idea, take some characters, and push them even harder, reveal even more. My own lacklustre book 2 was foundering a bit. The main thread was solid, the historical was going fine, but the second character was just splashing about in the shallows. He now has a solid plot of his own, that can bring him back to the principals and I get to do some research into a social behaviour that I find fascinating, if a little disturbing.
Around the world there are people who are either erotically, or more disturbing, psychologically aroused by drinking human blood. Some just extend sexual intimacy and the exchange of body fluids, and many play with vampire archetypes. I can't see that's any more worrying that any other submissive/dominant sex play that some people enjoy. Fifty Shades, anyone? No? But the idea of exploiting people who either feel a compulsion or need to feed on people, or those who for some reason are happy to be donors...I don't want to judge, and neither will my character, but there is potential for problems. Alternative lifestyle, creative sub culture, role play...great. Psychological dependence...hm. Anyway, it's a rich vein to explore, if you'll pardon the pun.