Here's a list of top notch books (imo) I have gleaned from here and elsewhere. I cannot vouch for them because I have not read them all.
Top Secret/Majic: Operation Majestic-12 and the United States Government's UFO Cover-up (Paperback)
http://www.amazon.com/Top-Secret-Maj...ref=pd_sim_b_3
The Day After Roswell (Mass Market Paperback)
http://www.amazon.com/Day-After-Rosw...6657613&sr=1-1
Ufos Are Real: Extraterrestrial Encounters Documented by the U.S. Government (Paperback)
http://www.amazon.com/Ufos-Are-Real-...7026956&sr=8-1
Dark Mission: The Secret History of NASA (Paperback)
http://www.amazon.com/Dark-Mission-S...pd_sim_b_njs_2
The Cosmic War: Interplanetary Warfare, Modern Physics and Ancient Texts (Paperback)
http://www.amazon.com/Cosmic-War-Int...pd_sim_b_njs_1
The SS Brotherhood of the Bell: Nasa's Nazis, JFK, And Majic-12 (Paperback)
http://www.amazon.com/SS-Brotherhood...pd_sim_b_njs_2
The End of Days: Armageddon and Prophecies of the Return (The Earth Chronicles) (Hardcover)
http://www.amazon.com/End-Days-Armag...6657730&sr=1-1