By Edward J. Ruppelt, former head of the United States Air Force Project Blue Book
Written in the 1950's this is an interesting account by the officer who ran the United States most famous investigation into the UFO phenomenon that rocked America in the late 1940's and early 50's, culminating in an unprecendent series of observations 1953.
There are no answers in this account as its primary purpose seems to be to account for Project Blue Book rather than attempt any explanation regarding the truth behind UFO's, but the report is full of curious examples of people who witnessed both the explainable and the unexplainable. What makes this account so compelling is both the mysterious aspect and the historical. After all this time, a great many of the examples set forth remain completely baffling
The book is available online here, which is where I read it as part of my research for Operation Herald.