1. Each book in the series should have a strong story. Whether the plot is elliptical or linear it should have a solid conflict and resolution. I know that might sound a little rigid but it is why we read books after all. Some semblance of narrative structure will give the book a feel of completeness. We don't read them to watch nothing happen or get solved. If the reader can't identify the central conflict of the story and how is was resolved, the book needs editing. Each book should be full of interesting or exciting events that arrive at some sort of conclusion.
2. The ending makes or breaks a book. Every book even when in the middle of a series needs a good ending. It needs to make the reader feel satisfied but also leaves them wanting more. I've read too many books that just abruptly end in a cliffhanger. This would be the wrong way to do it. Ideally the alternative would be to have a resolution for one major item and then set up a new conflict or intrigue that kicks off the next book. A lot of series books fall into the trap of continuing one major story line throughout several books without properly segmenting them into individual pieces. I understand the temptation is to make the reader feel more obligated to buy all the books by making them so closely linked where the only resolution they will get will be at the end, however if the reader likes your books they'll buy them anyway, no matter how well your hero saved the day in book one. Remember, the hero can save the day in books 1, 2, and 3 and not just in the final book.
3. The characters evolve. The great thing about series books is that you stay with a group of characters for so long that you can get incredibly attached to them and they can have the most amazing transformations and character development. Make your cast of characters diverse, interesting, believable and have them grow and evolve and you'll be golden. As cliche as it may seem people do often like it when good guys go bad, bad guys are revealed as good, enemies become friends and friends become enemies. The trick for making your twist seem non-cliche is to make sure it is shocking and not foretold.
Some other things to consider for your series:
- The book covers will be appealing if they are artistically cohesive with each other.
- The book should say it is part of a series and what number it is in the series clearly on the cover so the reader doesn't accidentally read it out of order.
- If you are repeating a story you have already told from another character's perspective it should offer a LOT of new and unpredictable insights.
- If you are a new author a free short story prequel book could help readers discover your series. Just make it clear that is what it is instead of startling them with a cliffhanger.
A couple series that I like:
The Julius Romeros Extravaganza by Hayley Lawson-Smith. This is a 5 star book series that has a wonderful cast of unique characters and a great story.
The Fever Series by Karen Marie Moning. A sexy, exciting series about evil fae with a protagonist that evolves beautifully.