All multi-book stories are part of an arc called a story arc. Imagine two people walking to the end of the block? bad cliffhangers are sort of like if you just stopped--sometimes as you're about to walk into traffic, sometimes not. A good cliffhanger has something you really want to know (like is that guy going to die because there's a car coming?) a bad cliffhanger just ends the story because the writer wants to break the a longer story into parts or didn't set up a Lady or the Tiger kind of situation. In other words--the hero just stopped and there's no car in sight.
Most stories have natural stopping points that work with your story arc where something happens or doesn't happen.
Like maybe the hero isn't invested vs the hero "is" invested.
Think of Han in the first Star Wars. Remember where he leaves and everyone thinks he's ducking out because he doesn't care about anything but money, then he comes roaring back to give Luke a chance to destroy the Death Star? That's because he's got to the road and he needs to look both ways, right? It's a natural stopping point or cliffhanger, because in Han's character arc he needs to realize that he cares about the people who've become his friends, and sometimes you have to stand and fight. That's why a cliffhanger works or doesn't work. If Han went off to do the Han Solo adventures and Star Wars went on with Luke crashing on the surface of the Death Star and Leia fighting on, then it's not a good cliff hanger no matter where the story breaks off because Han stays static.
Invested or not invested being just another way of saying he's reached the transformational point of his character arc and needs to make a choice. The whole story has set up his change--so will he or won't he? And the next story shows what happens depending on his choice.
The Rebellion fails because Luke didn't destroy the Death Star and everyone is blown up leading to a story where a single survivor starts the fight again on a distant planet? Or Luke destroys the Death Star and everyone argues about whether Leia knew she was kissing her brother?
A good cliffhanger needs to make sense for the story arc. It needs to give the reader a sense that the story could go in at least two directions because it's reached the climax of whatever arc you've decided to use.
(e.g. The story arc
The cliffhanger happens when they attack the Death Star. Will they or won't they win?
The character arc
(Han's) the cliffhanger happens when he gets in the Falcon and leaves the Rebel Base with the money. Will he or won't he fight?
That being said, this doesn't mean the end of the arc is the end of the story. The thing about cliffhangers is that it implies there's another arc.
There's going to be an Empire Strikes back, or a Single soldier escapes to start a new rebellion, or Han falls in love with Leia.
