The universe is chock full of Kindle Freebies and I have downloaded my share. But it is also true that you get what you pay for. And what you usually get in a Kindle freebie is an Incomplete book. I recently posted about "books without a satisfying ending". I got a lot of quibbling about "what do you mean by satisfying" and "whats the matter, are ya too cheap to buy the rest of the books?" No, I would gladly spend a few bucks on the next book if I liked this one. But I don't like incomplete books.
Every book should have its own complete story arc.The character should achieve their goal, even if there is a larger conflict loomng in their future. Let's make an analogy. LOTR is an example of 3 books that are incomplete in themselves. If you don't have all 3 of them in hand, forget it. (I remember the days when you couldn't look up sequels on Amazon. If you couldn't find it in the bookstore, forget it.) For an example of a series where each book is complete, I refer you to the Narnia books. I highly recommend these cute classics, at least the first 3, for examples of Complete stories with a larger story arc.
I just read a book where a kid lost his mother, discovered he had Powers, spent time in a school for kids with Powers, and got caught up in a conflict with cyborgs. This book felt incomplete because his questions about his mother and his powers were not answered. The author's strategy seemed to be "keeping information from us to create suspense so you would read the next books." In the end the protag did win a battle, but it felt incomplete because so many threads were left hanging. This strategy does not work for me as a reader: if I am served one incomplete meal, I won't go back to that restaurant.
It would take a skilled writer to structure this plot differently so that the 'threads' were wrapped up. The goal could have been to find and rescue his mother. He could have endured capture by the cyborgs, rescued his mom, escaped, and learned about his powers that way (instead of being boringly lectured about them in a school.) At the end of the book he could have arrived at the city where he would learn about his powers. A glorious, complete ending!
I once raised an example. Let's say your series is about defeating a great Evil: the Nazis. Each book could have its own goal. Book one: "escape from the concentration camp". Book 2. "win a major battle and rescue a spy." Book 3. "find the Nazi A-bomb factory and destroy it." Each book is a complete story in itself. If you pick up book 3, you can pick up book One later. Sure there's a backstory about the characters in book one and two. Just enough that you'll want to go back and read them.
An example from my own work. Book one, 'Tesla's Signal': Nikola Tesla defeats an alien invasion. Book two, 'Tesla's Frequency': Nikola Tesla defeats a Nazi takeover plot. Yet each work is tied together, same protagonist with same backstory and history (united states, 20th century.) If there is a Book 3 or 4, it would be about a future time where Tesla's disciples or descendants use versions of his inventions to save the world, Universe, etc. I wasn't sure if I wanted to write any more Tesla-themed books, but now I think I have to: just to illustrate what a series of complete books should be!