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The latest adventure of the Thursday Murder Club is also its best.

Richard Osman's fourth book in the series, "The Last Devil to Die," is another madcap mystery in which a quartet of surprisingly matter-of-fact octogenarians, residents of the same retirement community, use their unique sets of skills to solve murders (it helps that one, Elizabeth, is an only semiretired spy).

Osman's books get laughs from the specifics of his characters, who also include a pair of cops who reluctantly collaborate with the murder club and various thugs who are perhaps a bit more indulgent with them than real-life drug dealers and hit men would be. Even if the mysteries weren't absorbing — which they are — Osman's books, like Alexander McCall Smith's, would work simply because it's such fun to spend time with these people.

What sets "Devil" apart from its predecessors is the deftness and humor with which Osman confronts a subject that's completely not funny: dementia. Since the first in the series, "The Thursday Murder Club," the accumulating memory issues of Elizabeth's husband, Stephen, have played a role but they come to the fore in "Devil," when the couple must figure out their next steps — which they do in a way that feels logical and moving, even if it's unconventional.

In the end, the murder club books are not really about crime but about friendship and finding ways to stay involved in life, even if that means — as it does here — a missing cache of heroin and an alarmingly high pile of corpses with bullet holes in their heads.

The Last Devil to Die

By: Richard Osman.

Publisher: Viking, 352 pages, $29.