Albert Campion started his literary life as a seemingly extraneous figure in a curious book called _The Crime at Black Dudley_ (aka The Black Dudley Murder), where he inexplicably solves the crime rather than the hero. After that, he goes through a series of free-wheeling adventures -- ably assisted by his servant, ex-convict Lugg and Inspector Stanislas Oates. These books, a lot of fun and more thriller than detective oriented, include _Look to the Lady_ (aka Gyrth Chalice Mystery) and _Mystery Mile_. One of the most fun but least detective-oriented is _Sweet Danger_ (The Fear Sign, Kingdom of Death) where he meets a girl called Amanda.
Personally, I find subsequent Campion adventures less interesting, and Campion himself becomes rather a dull fellow. But during the World War II he finds himself involved in this wonderful adventure, _Traitor's Purse_, where he's on the trail of Nazi spies, but -- here's the trick -- he has amnesia, and doesn't even remember what his mission is, much less whom he can trust.
Anyone who knows Campion adventures -- as most readers would when the book was published -- will know where the loyalties of Amanda, Lugg and Oates lie, and perhaps it is good to go in with a few previous Campions under your belt to eliminate some of hero's utter confusion.
It's a great story and it moves like lightning.