"Flying Saucers from Outer Space" by Donald E. Keyhoe, 1953

It's a pretty interesting book, easy reading. What's striking is:

1) lame Air Force explanations for UFOs go back over 60 years

2) the argument about UFO's being extra-terrestrial versus secret US craft go back over 60 years-- Keyhoe presents why the US secret technology argument doesn't hold up and he strongly supports the ET origin of UFOs.