The book world is so saturated with series right now that they often get a bad rap. Trilogies in particular are often accused of being overlong, of creating three books where one or two would do. And that middle book usually takes the brunt of the criticism. I will admit that a lot of second books are slower than the first books and seem like set-up for the third books more than actual self-contained stories.
When an author has such a winner with a first book, it's hard to make that second one in the trilogy live up to its predecessor. I could name many, many authors whose second books were OK, or good, but not even close to the outstanding quality of the first. I could also name many whose second books were outright stinkers.
But that's not the purpose of this post. Instead, I want to focus on second books that soar, that exceed my expectations, that reaffirm my faith in series, which I have always loved. I've deliberately focused on trilogies that tell a single, unified story, featuring the same characters. This means you won't find books in the Graceling or Abhorsen series on my list (although I have opinions about those too). You also won't find longer series like Harry Potter. The list below features second books that were stronger than the first books, or at least approached the first books' greatness. These second books are far from filler.
Catching Fire by Suzanne Collins
I may be risking the wrath of my readers, but I think Catching Fire is a better book than The Hunger Games. It tells a much larger story, and much more is at stake than Katniss' life and the well-being of her family. Catching Fire is where we get conspiracies and the resistance and proof that what happens in the arena will have huge repercussions for the rest of the nation. It's tricky, too, because Collins makes you think it's going to be a rehash of The Hunger Games, and then she turns it into something different and awesome.
The Ask and the Answer by Patrick Ness
I loved The Knife of Never Letting Go, but The Ask and the Answer devastated me. After I finished it, I had to write out a hypothetical letter to Patrick Ness just to get all my feelings out. Where the first book was basically an extended chase scene, the second book is much more. This is where the trilogy really begins to delve into what it means to be at war, to subjugate other creatures, to be manipulated and to manipulate others. It also gives us the secondary perspective of Viola, which adds a lot to the story. This installment is just as strong as the first and third.
Red Glove by Holly Black
It's not quite as good as White Cat, but it's still a worthy successor. The actions of Cassel and the other characters carry over from the first book and inform the plot of the sequel, and the sequel does have a great plot: large body count, plenty of mayhem, a good mystery for Cassel to investigate, and lots of soul-searching. Cassel's and Lila's character arcs are also particularly well-done here.
Scarlet by Marissa Meyer
Technically this is a quartet, but I'm cheating and including it. Meyer helps make Scarlet a successful sequel by giving us another protagonist (the eponymous Scarlet), therefore giving us another perspective. Cinder is still there as a protagonist as well, but the addition of Scarlet grows the story. We see it from another angle, in another voice (though both are third-person), and we learn how the events of the first book affect more than just New Beijing. (I'll have a longer review of Scarlet closer to its publication date in February.)
There were a few sequels I've read recently that didn't make the cut: Ashen Winter, Insurgent, Dust and Decay. They weren't bad books, but they weren't close enough to the quality of their predecessors, and a lot of what was in them seemed like more of the typical filler that middle books get accused of. I also made a deliberate choice not to include The Subtle Knife, but only because I think it's the weakest of the three. (It's still a phenomenal book, but when it's bookended by The Golden Compass and The Amber Spyglass, it just can't compare.)
So, what ones did I miss? I'm sure there are more out there.