First of all, thanks for the kind words about the series. I really appreciate them. Glad you enjoy the series.
So, you ask a good question: how do we maintain a balance between action and suspense while keeping the story moving and not ending it prematurely?
Stories are built on promises. These can come in many forms: comedic, dramatic, horrific, etc. For example, if we start a story showing a woman in an ideal marriage with a husband who loves her and she’s satisfied and at peace, we’ve made a promise to readers: something is going to go wrong and disrupt her idyllic life. Perhaps her husband will die, or succumb to his old addiction to alcohol, or leave her, or have an affair. Something will throw things out of balance. If you drag it on too long before keeping the promise you’ve made, readers will get bored and annoyed.
You can make a promise by showing how blissful things are, or how unbalanced they are to begin with. So, we could have the daughter of a congressman get abducted—it’s a promise that the authorities will be in a race against time to save her.
One of the keys to storytelling is making promises that matter and then keeping them when the readers expect them to be kept. Suspense is really the drawing out of a promise of peril to a character whom we care about. It’s that apprehension we feel about impending danger. Action is what happens during the fulfillment of that promise. The action escalates to a climactic moment that drives the story forward into the next scene. Remember that in the interludes between scenes you will either need to keep a promise or make one.
A story ends when there’s no logical place for it to go based on the promises that have preceded that moment in the tale. If we make promises and don’t keep them, it’s likely that readers will feel frustrated at the end of the story. If we drag out a story after all the promises have been kept, readers will get bored.
So, as you analyze your story, ask yourself about those promises—have you made big enough ones on which to build a story? Have you kept the promises in ways that surprise readers and still please them? Have you done so at a time and in a way that will make readers clamor for your next book? You’ll need to stop and ask these questions throughout your writing process, but if you do, in the end you’ll have a well balanced story.