One misconception of series containing 6 books or more is that the characters remain at status quo. Or that the protagonists do not experience character development. Why else, they might think, would an author choose to write about the same characters so many times.
But that’s just not true. The protagonists of such series never stay the same. Whether it’s the protagonists words or deeds or just the author throwing in something the protagonist totally cannot control, what the main characters have done or what happens in previous books usually either pay off or come back to haunt the protagonists in future books. Hence, you usually find the main character either in totally different set of circumstances or in a totally different mindset several books later.
To my knowledge such multi-book series (the ones that contain 6 books or more) are usually found in crime / mystery fiction and its cousin, urban fantasy (which more often than not centers around characters that need to solve a mystery).
The start of the series is usually slow. After all, the first book (especially at the beginning) is used to introduce the main character and the world he or she lives in. But if the author is really good, you find out that the first book only serves to hook you onto the next one. And in fact, the first book is rarely the best book of the series.
Unlike trilogies where people often think of Book 1 as the best of the lot or most multi-season American TV shows, the book series which contains six or more books is more of an investment that gets exponentially more enjoyable as the series progresses.
Personally, I think the adventures (and consequent hardships) these series have put the protagonists through have made readers feel more sympathetic and more intimate to the plight of the main characters. In fact, even more so than trilogies.
Here are some examples that I think more people should check out:
- The Dresden Files by Jim Butcher (17 books and counting. I also suggest you check out short stories set in the same Dresden Universe)
- October Daye by Seanan McGuire (11 books and counting)
- The Hollows (aka Rachel Morgan Series) by Kim Harrison (13 books completed)
- The Kate Daniels Series by Ilona Andrews (9 books out at the time of writing this. With 1 last book coming out in a few weeks)
- White-Trash Zombies Series by Diana Rowland (6 books so far)
- The Kara Gillian Series by Diana Rowland (8 books and counting)
- Kate Burkholder Series by Linda Castillo (10 books so far)
- Harry Bosch Series by Michael Connelly (20 books so far)
- The Kinsey Millhone Series by Sue Grafton (25 books. Sadly, Sue Grafton passed away after the 25th book “Y is for Yesterday.” Hence, her alphabet mystery ends with Y)
- Imp Series by Debra Dunbar (10 books completed)
- The Nightside Series by Simon R Green (12 books completed)
- The Fever Series by Karen Marie Moning (10 books so far)