Of late, I've certainly seen a lot of them. Of course, that might simply be due to the waters I'm swimming at the moment. It seems everyone has, or is working on, or is planning a series. I see it like this: Picture the ocean. Each little ripple across that vast expanse of water is a book. The occasional hill or gentle roll of water contains the multitude of series - the bigger the hump, the larger the number of books in said series. Those smaller humps that might gently rock your boat consist of series number 2 to 4 books. The medium size humps, the kind that might make you hang on in your boat, or catch a cup sliding off the table, are made up of series numbering 3 to maybe 4 books. The big humps, the scary kind that might make you secure the hatches or simply get the flock out of there, are made up of those series of say 5 to 10+ books.
Now lets look at sales: When these waves make it to shore, they all swish up on the sand with a certain amount of foam or bubbles - those equate to sales. Most of those little ripples will generate some foam as it scrapes up on the sand, but not all. A series might curl over in a breaker, and then crash against the sand with a lot of foam and bubbles, but then they're spent too soon after. Now those big waves, the kind that attract surfers from around the world, those make an even bigger splash, but they also might scare some people off the beach - I do know I wouldn't want to hang too close to something like that, though I might be inclined to watch.
My point is, if I see an advertisement for a seventh book, and I don't find it interesting enough on it's own, I'll probably stay away from the entire series.
If you're writing a series, more power to you, but you never know who will find book X before they come across book 1. I look at the blurb, and if that's intriguing enough, I might take a look inside if I can. Given that sample, if I'm left wanting information from a previous book, I won't buy it, and I probably won't go in search of that first book. For me to do that, I have to be really intrigued.
My trilogy might be guilty of this very thing, but it was written as a single book that happened to fall into three parts. What did I know? At first it was over a thousand pages long, and that was just the text part. At the time, I was still in search of a publisher, and I was told that no publisher would take on such a large project from a newbie writer. So, still leaning on the experience of others, I offered the book as a whole, but conceded that it could be separated into three books. Book one made a pretty good splash. Book two, less of a splash, but then it was published in less locations. Book three was pretty much of a bomb. That is the last time I will publish a series one year between books.
I have another series in the works that is plotted out to be 16 books, but since most of them are really quite short, I think I'll only offer them as eBooks (kindle editions). I thought I might offer the book as a whole for paperback, but I'm thinking it might be too big. The biggest CreateSpace will handle is 828 pages. So if I do decide to go paperback with this book, I might release all books at the same time and sell them as a boxed set. Depending on ultimate page count, it might end up being three books. I'm working on book 10 at the moment, so I have a ways to go yet before I need to worry about any of that.
But enough about me.
What I'm getting at, is make each book of your series a stand along story, more or less. It's fine to have each story farther along in your character's timeline, but if your reader can't understand why Prince Caspian is staggering through the woods, going out of his way to avoid his guards, you might lose your reader as he drowns in questions.
Like I said, my series didn't do this. But I'm willing to learn from my mistakes, and above all pass along what I've learned, or at least my opinion of it all. The best series I've read did just that - each book told its own story, and while development might have occurred along the way, and book two is better for having read book one, book two and even book three were just fine on their own. I used to have the Tarzan series once. I don't even remember how many books there were, but they all stood alone. In fact, I think some of them were written by different authors, but I didn't pay attention. It was just another story, another adventure. I'm willing to pick up a book if I know it's the next adventure. Maybe that's why my trilogy doesn't sell so well.