I mentioned that I really enjoyed The Crown of Embers, but what I really want to talk about is why the second book in a series so often sucks, what can be done to avoid said suckage, and how this book actually fails to avoid most of the traps but doesn't suck anyway.
Some examples? Crossed, Insurgent, Magic Study. I liked Matched, Divergent, and Poison Study to various extents, but each of the sequels fell down, and I think in very similar ways. So you're an unknown author (all the ones mentioned so far were), and you're writing your first book. Trilogies sell. If the first book is good, people will come back to read more about their favorite characters. So you write one book, self-contained but with an open enough ending to leave you room to tell more story.
This means that to some extent or another, the first book needs to wrap up at the end and leave some open space, not just for new stories with the same character, but for more of the same story. Is this a weak point? Is a totally different adventure about the same characters better? Maybe that's it, but Catching Fire didn't have that problem; it closed off the occasion of the 74th annual Hunger Games neatly, but left us with the fallout from that experience and the same larger political scenario, then introduced the 75th games and turned things upside down again. And it did a great job with this.
So maybe you're ending on a cliffhanger, or a moment of transition, or maybe you have a temporary peace that you know will only be a lull, or maybe you just have more room to explore the problems in your world. The problem is that you can't build tension in a second book the same way you built it in the first. The elements that go into the beginning of a story--setting the scene, introducing the characters, introducing the problems, foreshadowing the troubles to come--some of these have been done already, some need to be changed or skipped. The proportions are going to be very different; depending on the strengths of the writer and the reader's relationship with the book, the proportions are likely to be a bit off.
(I'm thinking this through as I write here, by the way; this is not some frequently given, thoughtfully conceived rant.)
So you're building two stories: book one, and the trilogy. You need two sets of pacing, two arcs, two sets of character development to occur simultaneously. But as you're writing, you really need to sell the first book. So even if the first book ends on a cliffhanger, there's got to be some sort of emotional wrap-up, catharsis, payoff.
Which means that when book two opens, you're starting from a new "beginning." But you've already got your characters; likely you're going to bring in some more I've never met before. You've pushed down the bad guy; you'll either need a new one or some renewed threat. But wait, did you spend your best bad guy stuff in book one? Or did you leave your main character gasping in the dust, and now you have to spend a bunch of time picking them up out of the dust, brushing them off, and pointing them in the direction of the new adventure, all without character building (which has been done) to fill the time?
In that case, you're going to have to make their situation really horrible. Things were bad in the first book, but if this is going to be a trilogy, they're going to have to get worse. Is there anywhere worse to go? Better be.
I feel like there's some nugget of truth I'm not getting at here, some kernel of what makes a good and exciting book that a second book has a hard time pulling off. What did Days of Blood and Starlight do that was so good? I think the two conflicts--book level and series level--were present in the right proportions in the first book. It helped that it was a big chunk of flashback--the "present day" story dealt with the mystery, and then there were revelations to the mystery that gave you all this other story that was waiting there, that you were already invested in.
Did Crown of Embers do that? A little; winning a decisive battle leaves you at a turning point in a war that you probably haven't won yet, and also leaves you with a war-ravaged nation.
There are also, of course, the external truths, which are not the fault of the stories, but of the world they're being read in. First, finding a new book that you love is always surprising. A sequel will have a much harder time surprising you than the first book did, putting it at an immediate disadvantage. Second, a first time author probably spent some time span on the scale of years on that first book. The second one has been coming along, but now there's a timeline. Ravenous fans like me are salivating and you have to finish it NOW. There's no way you're going to have as much time to sharpen book two as you did book one.
This post was not as well-structured as I hoped it would be. A few days ago I thought I had a grasp on this elusive question, but I feel like maybe it was like one of those understandings that comes to you in a dream and then is gone. Ah, well; they can't all be life-changing inspirations. I hope my use of italics and caps lock has made up in conviction what I lack in coherence. Thank you and good night.