You've likely heard of "second book syndrome," but have you ever wondered why second books tend to drag and don't measure up to the first? I think about it quite often. How do first books with amazing characters, plot, and writing lead to boring sequels?
When picking up a new book, the summary draws you in first. It gives expectations of the book's plot, hints at character dynamics, tropes, and setting. Satisfaction from a story comes from whether the book met your expectations based on the synopsis.
The issue comes when picking up the sequel. You started the series because the premise, dynamics, and tropes of book one drew you in. The second book doesn't, and honestly shouldn't, have all the same elements. A good book at any point in a series should have a plot of its own. How does that work? Let's take a step back.
How does one even write a series?
Length shouldn't determine series vs standalone; you can't chop up your plot into three and call it a trilogy. Each book needs its own subplot, while also forwarding the overarching plot that threads throughout the series. This means problem x needs solving by the end of book one, or the reader ends up unsatisfied. But with x resolved, how does the series continue? Let's look at two common methods.
On the journey to solving x, you can hint at another issue, y. This issue cannot take center stage but needs building up in the background. For example, let's make x 'save the princess.' The hinted at y can address 'why does the princess need saving? who put her in this situation?' which would indicate a larger issue. This takes a backseat because the princess needing saving remains the immediate problem. Once resolved, your characters can address the 'why?', answered in the sequel.
The other method requires you to create a completely new problem brought up after the resolution of x. If not executed well, it reads like an excuse to turn a standalone into a series. It still needs hinting at, but instead of intertwining with x, use cause and effect. Let's use 'save the princess' again. The princess safely returns home, but during the rescue, the kidnapper gained valuable information to throw the kingdom into turmoil. This new problem couldn't happen until x's resolution.
Now let's circle back
Whether an overarching issue or a new problem after x, the sequel has a different plot. So, when a reader picks up a series based on plot x, you cannot meet those expectations with plot y. Plot x met its resolution, the characters have grown and changed based on their journey in book one, and many of those tropes don't come back. With the princess saved in book one, readers may find a sequel unnecessary. So then how do you keep interest and avoid the dreaded second book syndrome?
For one, character rather than plot driven stories typically thrive in series. Readers often pick up a sequel largely because of the characters and their dynamics. The reader wants to know how the characters' stories end more so than how the conflict gets resolved. In this way, the characters draw the reader into the sequel and keep their interest throughout.
What about plot driven books? Readers often don't read the synopsis of sequels; the enjoyment of book one alone carries enough weight to guarantee picking up book two. This means those readers go in with no expectations beyond their enjoyment of book one. Therefore, book two can't start out passively.
At the end of book one, your characters found themselves in a situation that warranted another plot. They can't sit around waiting for something to react to. Including action early on, and generally keeping things interesting, regardless of a different plot, can help keep readers' interest.
In the end, writing an interesting sequel matters more than anything else, regardless of what the plot is. You just have to remember readers may have different expectations than the author. You can't please everyone but having the right elements can go a long way.
Disclaimer: This post contains only my own opinions, based on analysis of various series that I've read. Not all advice applies in every situation, and there exists no "correct" way to write a book or series. The writing process differs for everyone :)