Is it easier to write the second novel after you've already published one? Is it easier to sell a second novel, after you've done your "debut"?
I can't really answer the second question, since I haven't started selling my second novel yet. I assume, maybe incorrectly, that having already had one commercially published novel out can only be a good thing, whether it was a bestseller or a door jamb. At least you've proven that someone else other than your parents or best friend or a vanity press (ahem, PublishAmerica, ahem) is willing to pay you for it. Still, how much does losing your publication virginity count these days? Do agents or publishers look beyond the fact that you're already a commercial novelist, and instead scrutinize your sales, etc.? Is being published by a small press beneficial or detrimental?
I don't know. I guess I'll find out some time soon.
As for the first question, I can tell you from my experience, it's HARDER to write the second novel. There is something freeing about working on your "first" because you're an unknown; nobody has ever read any novels of yours. They have no expectation. What you're doing with your debut is to set a bar, whether it's low or high. Certainly, you need to pass the test and the work has to be good enough to get published in the first place, but still, you're setting future expectations with your debut and the pressure is considerably less intense. Yes, you need to prove yourself and trying to get an agent or publish is trying, and many writers couldn't get past that stage at all. So, I'm not trying to diminish debuts. Still, all things considered, writing my debut was so much easier.
Why? Because with my debut, which I was very proud of despite its flaws (and how I wish I could go back and fix all of them), it was a modest effort. The plot was simple and the story was straightforward. It was more of a learning experience for them than anything else. I didn't even thing I would get it finished, let alone published. The experience not only exhilarated but also humbled me. I realized I had still so much to learn, and I also realized I had a lot of room to grow as a writer and a published author. I also set the bar pretty high for myself.
So it's all about expectations. My readers, those who actually liked The Pacific Between, surely will be looking forward to something better in my second effort. And I can't let them down. I can't let myself down. This second book has been difficult for me because while I "proved myself" as a writer with The Pacific Between, I feel the stakes are much higher this time: I'll have to prove that I'm also a good storyteller, and someone who can keep doing this, and getting better with every book, that my peak has yet to come.
Unrealistic? Perhaps, but I think it's a good sign. The sophomore effort is usually harder because of this expectation -- you have yet to prove yourself as an artist, but you've also set your bar at a level under which you cannot return. The last thing I want to hear is "this is worse than his first book." That'd be dreadful.