You might have heard of the phrase "World Building" in relation to novels. It's a term used to create a whole new world within the pages of a book, where something that happens in one novel affects the next book, and so forth. You have to have continuity. For example, you can't write something in a later book that totally contradicts what happens in a previous novel. This makes things harder for authors who writes series, and because of this they have to think about everything that they put in, so they don't get tripped up further down the track.
I like to write series, my first being "Behind the Lives." Partway through writing "Behind the Hood" I decided that I wanted to continue writing about Claydon and its residents. In "Behind the Tears," the second book due out in 2012, I decided to carry on the story by focusing on the Rata and Connor families instead of the Harrises and Daniels. But because of this I went back to change a few things in "Behind the Hood" prior to it being published, so that it would fit in with the sequel. I don't think many people will pick up on my clues in "Behind the Hood" about Corey and Sledge, but once they start reading "Behind the Tears" they should go, "Yeah, that's believable," because of how those two acted in the first book." Therefore, things shouldn't come out of the blue.
Also, Ash's background and a little of Dante's was pointed out in book 1, which is developed to a greater extent in book 2. Though, for the people that enjoyed the main characters in "Behind the Hood," don't think I've dropped them and that's it. In book three, "Behind the Lies," Nike and Jess return along with Leila, and from her comments and Jess's thoughts in their last chapter from "Behind the Hood" one should get a pretty good idea on how much trouble is in store for them.
Furthermore, as I write new books in the series additional characters are added to the mix. Their inclusions in previous books may be small, but the reader does gets an introduction, something that allows the reader to warm to them quicker.
"World Building" is also about progression, a carrying on with lives. Things aren't static in the world, therefore each book in the series can't go over the same territory. The characters should get older and they will undoubtedly come across new people. And as they get older their perspective on things will change. What seemed so important to them in one book may start to wane and disappear in another.
And as said, if an author wants the reader to continue to read their work they can't just keep giving the same story over and over again--just with different characters. I used to like reading Christine Feehan's writing, but got pretty sick of the same formula being used to the point that I couldn't be bothered reading them because I knew what was going to happen. Life isn't a carbon cutout.
All up, series should be considered as one very large book, not seperate ones.