This book has been surprisingly popular. I'm sure having a famous author doesn't hurt its chances in that regard, but it's interesting to see what gets the visibility. I mean, it's a pretty fun book, but not exactly great literature. How did I come to read it? My son got it, my wife read it, and I was looking for something light to read while going through medical treatment. For that it worked very nicely.
It's very readable, and the plot is enjoyable. The titular club is a group of septe/octogenarians who are spending their time digging through old murder cases and trying to solve them... when some actual murders turn up in their retirement community. The characters are larger than life, with a particularly overblown narcissistic and vacant property developer. The writing style of the book is very chatty; while most of it is in the third person, the text tends to align with the current character. Reading it is a bit like being set about by a garrulous bunch of retirees.
It's not pure saccharine. The retirees are active not because they're in a Miss Marple world of immortal little old ladies, but because they're aware how close the nursing home is (metaphorically and physically), and are making the most of what they have - genteely raging against the dying of the light.
One advantage of setting a whodunnit in a retirement community is the amount of room you get for long-buried secrets and history. The book is very willing to chuck in twists and red herrings, and it pulls in a few historical murders too. The result is fun. Not particularly believable, not incredibly subtle, but fun.