The maximum number of loose ends is four .... kidding, it's three. A few loose ends are okay, as long as the primary story feels complete. Don't be obvious about leaving antagonists to wreak vengence another day. Your readers will know they are still alive without you making it obvious.

Each book in a series should be able to stand on it's own and not rely on the book(s) before. You never know which a reader will pick up as their first and you don't want them lost and unable to make sense of it because they don't have the background to understand the subtext. Because of that, it doesn't matter whether you bring back antagonists from the first book, or present new ones. Never be heavy handed in anything you do. Readers are bright. They will figure things on their own.

Good luck -Craven

Well, I don't know about rules, but I can tell you that as a reader, I personally need to feel that there is an actual ending to a book. If there is an ending but maybe there are a couple things that didn't exactly get nicely knotted, I'm going to feel satisfied while still wanting more. If, as in the case of "The Hitchhiker's Guide To The Galaxy" I turn the page and am shocked to find that I have just read the last page of the book...I am NOT going to be happy. I'm going to feel tricked into getting the next book. But that is my personal pet peeve.

Ironically, I'm facing the same situation with my own writing. I set out to write one novel and discovered the story in my head just wouldn't fit into only one book so I'm very carefully considering where to place the endings and how to juggle scenes around to make it all work. I've got about three books worth planned out so far.

 

The flip side to this is how do you keep the thread going into the next book without spending chapters and chapters bringing the reader up to speed.  Some readers won't read the third book before the first but I have done it more than once.  I read a lot of Robert B. Parker and Grafton and they do an excellent job of sort of bring the reader up to speed with the character without spending half the book rehashing Spender or Kinsey's past.  It's artfully done (for the most part) throughout the book.

 

Of course, Raymond is right about the loose ends.  How he found out about the rules in Iran - I have no idea.  I would never have pictured him as one of the bearded ones.

Stand alone novels are the best way to go.  Later, as you sell other novels that may be related in content or characters, a trilogy or a series can be the goal.  As you develop a following of readers, they will want to see and read all of your novels and a trilogy or a series will satisfy that need.

Bob