The book is notable because it was, for several subsequent decades, the only account of Air Force UFO studies written by a participant. It remains arguably one of the most level-headed books about UFOs; Hynek suggested that Ruppelt's "book should be required reading for anyone seriously interested in the history of this subject". In the book, Ruppelt detailed his time with Projects Grudge and Blue Book, and offered his assessments of some UFO cases, including a portion he thought were puzzling and unexplained.
