Death in the Clouds by Agatha Christie
Twenty-one people have boarded the Prometheus at France's Le Bourget airport for a routine, one-day flight to London. But what begins as an ordinary excursion over the English Channel turns into a most extraordinary case of murder when a female passenger is shot with the venom-tipped dart of a South African blowpipe. Even more bizarre is that the killing could go completely unnoticed by any other passengers--including Hercule Poirot, the brilliant Belgian detective, sitting not fifteen feet from the victim.
This was another good read from Agatha Christie. I don't think I've read a single book by her that I didn't like. It wasn't my favorite of her books, but still suspenseful and well-written as always. I thought I had it all figured out, but, as happens most of the time with her books, I ended up being wrong in the end!