Usually the second book falls short.
Here are some common themes/issues I have seen with the middle book of a series (and something as a writer you should be conscious of):
- The Love Triangle. You know ... where the main character suddenly falls head over heels for that other person. It's an easy way to build tension in the relationship ... but it seems to be way overdone IMO.
- The Breakup. Or what I'll call the Twilight syndrome. This can be the love interest breaking up with your MC ... or the MC thinking the love interest is better off without her, so she does the breaking up. Either way ... lots of angst.
- The Paranoia. And yet another (annoying) way to add tension between your couple ... the MC thinks the love interest is constantly cheating on him/her. Every move, every word is over-analyzed.
- Book 1, Take Two. My latest example of this is Unravel Me (which I know isn't out yet, but I got the ARC at BEA over the summer and I feel like that was eons ago!) I don't want to spoil it, other than to say that I feel like the character growth was exactly the same as Shatter Me. Juliette somehow went from being this strong character, no longer self-editing her thoughts ... right back to square one.
- The Tangent. You've established a world in the first book. And then BAM your characters are off somewhere completely different with different rules, settings, and you are practically starting over. It seems a bit of a cop out to me. And it doesn't make me want to keep reading. If I liked your first book enough to pick up the sequel, I kind of want to stay in that world.
- The Cliffhanger. This isn't so much an issue with the plot, and it it can appear in the first book too ... but I despise the cliffhangers. May be a personal thing, but goodness do I hate them!
So there you have it. My issues with sequels and trilogies. Sure, there are some authors I love enough that I'll pick up all their books, no matter what. But they are few and far between.
And just in case you're interested - one of my favorite sequels of all time (and it was a surprising one, because I didn't so much like the first) was A Million Suns. Beth Revis managed to continue the same romance (without much drama between the two characters) keep us in the same world, and yet throw a new spin at us. It was a great plot, and a great bridge (hopefully) between the first and last book!