JD Robb has written 53 books in the In Death series in the past 21 years. That’s a huge number of books. (Some of these are novellas that appear in anthologies, but it’s still impressive.)
The books center around Eve Dallas, a police lieutenant in the New York Police Department, and her husband Roarke, multi-billionare businessman and former criminal. There are also multiple supporting characters that make regular appearances.
Each book centers around one or more murder that Dallas must solve. As the series has continued, Roarke assists her more and more often.
I love reading these books, but I’ve also started studying them from a writer’s perspective. If you’d ask me, I would have told you I didn’t think that a character arc could span over 53+ books, but I would have been wrong.
Dallas and Roarke have continued to develop, as a couple and as individuals. Though the focus tends to be on them and their relationship, the other characters in the universe are interesting and often experience character growth of their own. I love the fact that a married couple can continue to be the subject of a series; too often the curtain drops just after the wedding, but that’s not real life. They argue, they compromise, they have past lovers, and yet they navigate it together.
Each story shares characteristics, but they’re not formulaic. There are multiple series subplots, like Dallas’s past, and information about these is doled out over time. It’s masterful the way Robb keeps my interest in these subplots. She drags them out for just the right amount of time so that they never get stale, but also never turn into an info dump.
The books are thrillers with elements of romance and science fiction, and while the science fiction might not please hard-core sci-fi fans, they’re always a good story.
More than anything, I want the stories I tell to be compelling and interesting. I think that’s the best rule for any author: tell a good story.
“Life is never as long as we want it to be, and wasted time can never be recovered.”
― J.D. Robb
Doree Weller 
Mystery series are known for having detectives who don’t evolve much as characters, so it’s refreshing to hear that this series offers characters with arcs and development.