One of the things that is so appealing about book series is that a good story doesn't end quickly. And there are certainly books that we have not wanted to end. Most of us have read at least one book in our lives that left us wanting more. No matter if the plot had played itself out fully or not, we just didn’t want the story to end. We wanted to stay with the characters a little or a lot longer. These are some of the best books. With a series, we get at least one more chance to dive back into the story that engrossed us so much the first time.
Another thing that may be appealing about book series is that the story can be far more in depth than your average-length novel. A lot of detail can be added to a lengthy series that may have been left out had the story been shorter. This can mean longer backstories for characters, better descriptions of settings or more subplots. Of course, if the series isn’t any good, then this isn’t as much of a positive, but if the series is fantastic, more books is a good thing.
Book series are also appealing for the very fact that they are more than one novel. It can be difficult to find a book that is interesting sometimes. If you have already read one or more of the books in a series and enjoyed them, chances are you will enjoy the next installment. In this sense, you get to skip the lengthy decision making process that often happens at the bookstore by simply continuing a series.
Series I recommend:
The Vampire Chronicles by Anne Rice
The Harry Potter series by J.K. Rowling
The Dark Tower series by Stephen King
Dune and the related books by Frank Herbert
The Hitchhikers Guide to the Galaxy series by Douglas Adams (These are all included in the "Ultimate Hitchhikers Guide to the Galaxy.")