Above Top Secret The Worldwide UFO Cover-up by Timothy Good is probably the most comprehensive book in the field. 

The New York Times says, "The hottest topic among UFO enthusiasts is what they describe as the Federal Government's experience with aliens...Timothy Good and a group of UFO investigatiors in the United States say they have documentary evidence that the government hid its knowledge...The chages are contained in Mr. good's book Above Top Secret.

This is a valuble contribution to UFO literature and a fascinating read, because it is well written, carefully researched, and packed with amazing accounts of UFO sightings