Some more experienced writers have told me that writing the second book is much harder than the first.
So far, with Thorn of Dentonhill II (new working title: Elements of Aventil), that hasn't been the case.
Not to say I haven't had my challenges. It's gone steady, but around the 2/3 point, my writing output has slowed down. Not stopped, but gone from 5-6K a week to 3-4K. But that's not uncommon. Every book I've written has had some sort of mid-point quagmire, and frankly, this has been the least troubling one to date. I mean, Way of the Shield spent months in around 30,000 words where I just couldn't figure it out. So spending a week longer to hit 70K on Elements is hardly cause for alarm.
But why do writers have trouble with second books? Mostly this comes from the fact that first books usually have years of midwifing, where writers work at their own pace. Polishing and perfecting. So once that finally sells, the pair of questions "Great, what's next? And can you have it done in six months?" can certainly seem daunting.
Now, perhaps why it's less daunting for me is because I kept working on rough drafts of new material while shopping Thorn-- a tactic which paid off, since I sold Thorn and Murder of Mages at the same time-- so the process of "Now how do I write another novel?" was demystified. I worked out a process of outlining. I've been coming at Elements with this solid outline-- which has, for the most part, survived intact, though there are plenty of aspects finding their way in that had nothing to do with the outline.
Of course, part of this has been to address the question, mostly imposed on myself: now that I'm "a professional", can I grind out the work at the pace I need to? Part of the reason why Way of the Shield could sit and do nothing for so long is that no one was really asking for it. In theory Elements of Aventil is going to have people asking for it, and having it done, proving I can do it as fast as I need to-- especially now, when we aren't at "quit your day job" money yet-- is an important next step.
So, in brief: yeah, writing the sequel is hard. But, fortunately, it hasn't been as hard as I feared it could be.