On Nov. 25-26 I'm going to run another free book promo for Paths of the Shadow, with the goal of getting more people introduced to the series. In a way, getting readers hooked to a book series can be like pushing dope: you offer some for free at first and, if it works, people pay their own way later.
Series can be tricky this way: on the one hand, once you have a loyal reader who has read the first book in the series and become enthusiastic, the sales for the rest of the books are pretty much in your pocket. On the other hand, a lot - and I mean a lot - hinges on the first book: if it fails to grab the readers, it doesn't matter how brilliant subsequent books are - most people won't bother reading them; although it may occasionally happen that people stumble upon a middle book in a series and start from it. Back when I was introduced to Harry Potter (many years ago) I first came upon book three, started reading that, and then went back to book one.
Another catch with series is the need to keep providing good-quality books with a cohesive storyline. I have known several instances when a series started well, but latter books were clearly drawn-out with the idea of generating more books and more sales. This irritates readers and makes them feel cheated. It is important to know where to stop; better to have a good-quality trilogy than ten endlessly stretching books.
Also, the author of a series must be consistently productive or risk losing readers. Even (and especially) a world-famous bestselling author such as George Martin receives a lot of criticism from disappointed readers who feel they have waited for the next book far too long. Most authors will have reader simply tune out, forget about the series and move on to something else.
Having said that, I think series are wonderful, especially for fantasy. They allow for intricate world-building and make a terrific platform for reader engagement - if done right.