Many story experts will tell you that not only does the plot need to be resolved at the end of the book, but the main character must go through a transformation as well. If a plot is resolved, but the main character makes a change that is just one step on her transformative journey, you can continue to explore that journey in the next book (and if you’ve done it well, your readers will join you).
In addition, a plot can be resolved to a reader’s satisfaction without all the loose ends being tied up—and one of those loose ends can lead to the central plot of the next book. For example, if the detectives catch the killer at the end of book one but didn’t catch the mole in the police department who gave the killer inside information, that mole could be a driving force in book two.
The Harry Potter series is a wonderful example of structuring books for sequels that each has a satisfying ending. Harry goes through a typical “hero’s journey” in each book (even going to a metaphorical or literal “underworld,” like Odysseus did, in each of the seven books), with the central question of the book resolved at the end. But the overarching question—can Voldemort be vanquished (and from a character arc perspective, can Harry be transformed from a wizard unaware of his powers at the beginning of book one to a powerful wizard who defeats the Biggest Dark Wizard Ever at the end of book seven)—is a master class in how a writer can overlap a book’s plot with a series’ plot/theme/structure. Each book builds Harry’s knowledge, bit by bit, with things that he absolutely needs for each step of his journey to defeat Voldemort.