How pervasive is the “Thursday Murder Club” universe? Consider this: The fifth book in the bestselling series, “The Impossible Fortune” (out Sept. 30), includes a joke about an actor who stars in the movie version of the first book, which zipped to the top of Netflix playlists last month.

That Pierce Brosnan inside joke in “Impossible” was obviously planted as an Easter egg for fans of the movie based on the novel “The Thursday Murder Club,” which debuted in 2020 and has produced a sequel each year except 2024, when writer Richard Osman published a non-“Thursday” thriller called “We Solve Murders.”

But if you’re not already one of those fans who knows that Joyce loves to make lemon drizzle cakes or that Ibrahim’s sexuality has some question marks attached, here are a few factoids to help you feel like you’re part of the club:

What is the Thursday Murder Club?

The movie truncates the origins story of the four, who all live in a deluxe retirement community in England. They are led by canny former spy Elizabeth (played by Helen Mirren in the film) and also include scattered-but-surprisingly-insightful-about-people Joyce (Celia Imrie, who’s much less scattered in the film version), strongman Ron (Brosnan) and nerdy ex-psychiatrist Ibrahim (Ben Kingsley).

One of the treats of Osman’s books is his keen insight into the differences between these four, and how those differences make them an even stronger unit, almost like the “Avengers,” with calcium supplements instead of superpowers. Joyce, who narrates many of the chapters in all of the books, has a particularly charming point of view, which veers between being baffled by modern technology and intuiting exactly how crooks might think.

Why are they always getting mixed up in murders?

Their first case involved the death of one of the owners of their retirement community, so they were already on the scene and had inside info that helped them figure things out. Since then, Osman has skirted the “Murder, She Wrote”/is-anybody-still-alive-in-this-small-community syndrome by strengthening the club’s ties with both cops and robbers. They’re good friends with an officer named Donna (Naomi Ackie plays her in the film), who funnels info to them and benefits from their insights. And they’ve developed relationships with a few criminals, whose exploits pop up across the five books (Ron, officially a retired labor organizer, has some unofficial criminal connections).

Do I need to read the books before I watch the movie?

Short answer: “Need” is a strong word, but maybe? Osman’s books — “Thursday,” “The Man Who Died Twice,” “The Bullet That Missed,” “The Last Devil to Die” and “Impossible” — are absolute delights. There’s a bit of violence, but the meat of the books is the relationships between the characters (also including those police officers and various relatives and miscreants).

Although Osman was a producer on the film, his books are better and they are significantly funnier, with several wry wisecracks per page. These are quibbles, but I’d also ding the movie for too-obvious musical choices and for sloppy casting: Jonathan Pryce, who plays Elizabeth’s husband, also plays a spy type with dementia in “Slow Horses” and Daniel Mays, who portrays a blustery cop, plays a virtually identical doofus on PBS’ “Moonflower Murders.”

Some mystery series are not especially adept at character growth, but Osman’s books are. Joyce and Ibrahim, in particular, have been through periods in which they have felt invisible, but both blossom over the course of the five books. Ibrahim probably has the smallest role in the first novel, for instance, but a key relationship he develops is at the center of the new one, “Impossible Fortune,” and it’s a hoot to see how the dynamic between him and a crimelord named Connie has shifted. When I watched the movie, it had been six years since I read the first book, so it was intriguing to recall that one person who is a major suspect in “Thursday” is now a key ally of the club.

Are all of the books good?

I wasn’t as wild about Osman’s non-“Thursday” book from 2024, but I have loved all five “Thursday” books. I will say that you’re more likely to appreciate them if you’re drawn in by the characters than if, for instance, you are a hard-core mystery fan.

Osman does a decent job with the puzzle aspects of the homicides that are solved in each of the books, but he sometimes gets bogged down in the details. That’s especially true of “Impossible,” in which the cryptocurrency-related murders are one of several plot threads that eventually wind together. I tend not to care if things get a little too twisty because the fun is in hanging with these people more than in the specifics of whodunit.

Will there be more movies?

No sequels have been announced, but the first movie is a huge hit and there are four more books just waiting to be adapted. With a cast whose leads range from 72 (Brosnan) to 82 (Kingsley) and who are busy with many other projects, there’s no time like the present.

The Impossible Fortune

By: Richard Osman.

Publisher: Pamela Dorman Books, 359 pages.

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