When I was a kid, I liked series
books like Nancy Drew or Encyclopedia Brown - a series that was really a collection of books or stories bound together by a central character. There were already a lot of completed books waiting to be read, and it didn't really matter what order I read them in. But today it frustrates me no end to stumble onto a series in progress. You have to wait at least a year for the next book, and with a lengthy series you have to struggle to find the patience to wait for the next and the next for years to come. Really - how many kids can hold on that long? At least the Harry Potter series grows with its readers. A ten-year-old who discovered Sorcerer's Stone and grew up with the series found herself only a little older than Harry by the time Rowling completed the series.
I was fortunate to come upon the Harry Potter books partway through the series' publication process. Although it was agony to wait (relatively) patiently for the final books, I was able to read the first several straight through, one after the other without pause. And each book was complete in terms of plot, building to a satisfying resolution, even though the complete series built to one overarching plot beyond the individual books. What a delight to discover that minor details in Sorcerer's Stone had their payoff in Deathly Hallows!
But few series are that satisfying. The plotting and writing are rarely of the same quality in each book. Even when individual books are excellent (such as the first two books in The Hunger Games series), the whole series fails on its payoff when the final book ends up relying on twists such as character inconsistencies that diminish the whole. And even though teen readers might have grown up with Bella in The Twilight series, they were betrayed by the breaking-down of the resolution pattern in Breaking Dawn which hurt the series as a whole.
Chapter book series and some middle grade series are satisfying, both because they are shorter and written (and therefore published) faster, and because most of them rely on the Nancy Drew and Encyclopedia Brown series rules: the same or similar characters in each book, with plots that can be enjoyed in any order, not leaving readers desperate to find out what happens next.
YA authors, please consider writing more stand-along books. A sixteen-year-old reader who falls in love with the first book in your series will be in college and possibly graduated and married before the series is over! Her reading interests will have matured and, unless your writing skills are on par with Rowling's, she may never make it to the end of your series. However, if you write a single perfect book, she will read it and may never forget it. Isn't that what you really want?