Shermer is a science writer and skeptic. His writing is engaging, and very accessible to those with little background in his chosen topics. I'd recommend starting with Why People Believe Weird Things. He examines why intelligent people embrace strange/unsubstantiated beliefs (alien abduction, Holocaust denial, creationism, etc). It's very much in the vein of Sagan's The Demon-Haunted World. There's an interesting chapter about Ayn Rand.

A great read, but best to leave this book until you have become more familiar with Christie. It is detective Hercules Poirot's last case, and you'll enjoy it more if you are well acquainted with the character.