Probably the second book. During the first book, you're still on that adrenaline rush. The third book (if you've planned your ending at least) is the big climax, so it's just a small matter (ha ha ha ha) of tying of loose strings.
But, oh, that second book. That dread middle!
Elana Johnson has a great post on this:
I think Book Two is the hardest. Let's just get that out in the open up front. The author has the challenge of living up to Book One, and the characters aren't new. The world isn't new. The problems aren't new. We got to see all of those things in the first book, so Book Two usually suffers from Little Sister Syndrome. In fact, in my exploration of trilogies, I read many (MANY) a second book that I felt was exactly like the first. I felt like I'd read the same book twice.If your first book has already been published and found some fans, you're problem is worse. Now you have the added pressure of pleasing your fans as you sit and write. Argh. This can feel like you have a crowd peering over you shoulder as you type, and there's no faster way to hit writer's block. Mike Mullin discusses this in an interview with Lissa Price:
LISSA PRICE: Mike, your first book, ASHFALL, had tremendous awards and honors. You were one of NPR’s top 5 YA novels, and Kirkus had you on a Best Teen Book List as well as a starred review. There were many more honors. How did any of this affect you as you wrote the sequel, ASHEN WINTER?
MIKE MULLIN: It has certainly increased my writerly anxiety. The question is always hovering just out of sight behind my left shoulder: Can I write anything as good as the first book? Answering that question will be up to my readers, of course. I'm not the only one who thinks so either.A trilogy is the three act structure divided into novels. The second act is like the second line of a joke...the repetition of a pattern that will finally be broken in the third act. There is some repetition, therefore, inherent in the structure. While you don't want to simply repeat everything that happened in
Book 1, you have to repeat some elements. How can you maintain this balance?
Here's a quick and dirty list of seven tricks to get through the second book. Not all are necessary, of course (except the first, higher stakes--stakes should rise continually throughout a series).
1. The stakes are higher.
2. The protagonist is more powerful now than in the first book.
3. The protagonist loses a major ally.
4. The protagonist must make an unexpected alliance.
5. The love interest loses out (temporarily) to a rival.
6. A (false) victory for the hero. The reader knows it's not over, but not the hero.
7. A (false) victory for the villain--a dark night of the soul--in which the villain seemingly prevails.