Part of that being Dresden books are easier to write then his non-Dresden because Dresden is written in first person and his other series are not.
I agree and disagree with that sentiment. I don't find first-person inherently easier to write than third. I find third simpler in a lot of ways—I don't have to worry about restricting narration ("I'd like to say
this, but the perspective character doesn't have access to this information"), I don't have horse blinders on in terms of plot (I can bounce around between characters, each of whom have different things going on, different information, different personalities, etc., so I can keep things going without everything happening at once, in an unbroken chain of events), it's easier to manage pacing, and—this is a big one for me—I don't need to keep all of the characters together to prevent things from happening off-page.
Basically, first person is "easier" in that the actual prose you write can be much more casual (or not, depending on your perspective character), and it can help you stay focused on a tight narrative. But third gives you so many more narrative options that it can be a whole lot simpler to tackle tougher plot points (and it's helped me avoid a bad block, more than once). I can jump to a different perspective character when the plot starts to drag a bit, to keep things moving along nicely.
On the flip side, it can be harder to keep track of each character or group of characters in longer, complex stories. You'll notice in the Codex Alera series, Jim stuck to a small cast of perspective characters, pretty much sticking to three or four in each one—Tavi, Amara, Isana, and Fidelious (sometimes), plus an occasional villain. That kept things simple enough that he could use the perspective's greatest strengths, and limit the biggest weakness. Aeronaut's Windlass was more troublesome, with... hang on, it's been a while—um... six? Seven? Folly, Grimm, Gwen, Bridget, Rowl, and that Auroran Marine, what's his name. Maybe one more. Anyway, that might've complicated things a bit.
I wrote one door-stopper book with about twenty perspective characters that was a total blast to write, in large part because it bounced around so much. But it was a story that I knew extremely well before I sat down to write any of it, so I didn't have much trouble tracking the different character journeys.
Basically, what I'm saying is this: it's entirely possible that the third-person perspective is part of the break for Jim. First can be really restrictive in a lot of ways, and sometimes it's fun to just cut loose and do what you want.
