Everything old is old again in Richard Osman’s latest comic mystery, “We Solve Murders.”
It’s not part of Osman’s “The Thursday Murder Club” series, but his fifth book to feature detectives who are old enough to collect pensions and read Modern Maturity. And it’s part of a wave of mysteries with sleuths who are more likely to pore over clues while clicking away at their knitting than to race down an alley after a maniac.
Many current series feature older detectives, so it’s a legit trend. But it’s not exactly new.
These contemporary books, including beloved series by Jesse Q. Sutanto and Alexander McCall Smith, follow in the footsteps of the legendary Agatha Christie. Her Hercule Poirot was already a retiree when he made his debut (“The Mysterious Affair at Styles”) in the 1920s. And Miss Marple was described as quite elderly when she appeared in her first novel, “The Murder at the Vicarage,” in 1930. (Both continued to detect for more than four decades, barely aging in the process.)
New, older detectives are popping up all the time: Brothers Ian and Will Ferguson just released “Mystery in the Title,” which features a has-been TV actor who keeps stumbling over corpses. And next June a series debuts with a Jamaican British woman who’s retired from nursing but not from solving crimes, “A Murder for Miss Hortense.”
Here are five series, in order of preference, with gumshoes who, like Miss Marple and “Murder She Wrote’s” Jessica Fletcher, are lifelong learners. Because these characters have so much character, the books often de-emphasize crime solving and pump up humor and human interaction. So, we’ve included a ratio to help you determine how much mystery to expect:

The Great Hippopotamus Hotel
Alexander McCall Smith
Who’s detecting? Precious Ramotswe, a “traditionally built” Botswanan who operates the No. 1 Ladies Detective Agency, along with assistant Grace Makutsi. They’re self-taught detectives, so it’s a good thing this sweet, humane series finds them investigating not murders but why a daughter is suddenly ill-behaved or a small business’ ledger no longer balances. (Available now.)
Ratio: 20% mystery/80% African life.