Just finished "The Big Four" by Agatha Christie. Her 8th book, and the 5th featuring Hercule Poirot. Originally published in 1927.
Christie is best known for her crime fiction, but she also wrote many "thrillers", generally with wildly improbable plots featuring sinister international conspiracies and "master criminals". This book is (thankfully) her one and only attempt at inserting one of her regular detectives into such a thriller, and it just doesn't work.
The basic premise here is that a gang of four super-criminals (an inscrutable Chinese Mandarin, an American millionaire, a French scientist, and an assassin who is a master of disguise) - the "big four" of the title - are planning to take over the world (quite how is never made clear). Poirot, with his faithful sidekick Hasting (the narrator of the book) gets on their trail and sets out to foil their dastardly plans.
This is essentially a book of Poirot short stories with an extremely silly linking theme. An example, and a small spoiler:
I honestly cannot recommend this except to the Christie "completist". Thankfully she never did this again.