Blue Book was the name of an official UFO research project, conducted the US Air Force, with the assistance of some independent experts, such as Prof. Dr. J. Allan Hynek (+ 1996). It ran from 1951 till 1969.Blue Book was not the first such UFO research project. It was officially preceded by the Projects Sign and Grudge. (Some researchers claim that Gleem and Pounce were similar projects, but those would have been covert investigations).The Blue Book Report became quite famous. Terms like 'distant encounters, close encounters of the first, second and third kind,' all come out of Blue Book.
Nowadays, most sources agree that Blue Book was more of PR effort than a serious investigation, in spite of the efforts of Capt. Ruppelt to conduct an in depth enquiry. Ruppelt himself identified several cases in which information had been manipulated, or where he was denied access to relevant information. After he was taken off Blue Book, Ruppelt speculated that he may well have been the front man for an elaborate cover up. The late Professor Hynek admitted later on in life that his assignment had been to specifically "debunk" as many cases as possible in Blue Book.
When Blue Book was presented to the Press, misinformation was given to those attending the Press Conference. The Press Release mentioned that only 3 percent of the sightings remained unexplained and that the USAF was confident that an explanation for each of those could be found. There was only not enough information to choose the correct explanation.
In the official version of Blue Book, chapter (or "special report") 13 is missing. There has been a lot of speculation about what it contained. One source, who claims he has read it, says it dealt with close encounters with aliens, with abductions, with human remains found at UFO crash sites, and with mutilation cases.
Some sources say that Project Blue Book actually was a 'decoy' to draw people's attention away from the real research projects that were being done by the Military. WM Cooper, e.g., claims that Blue Book was organized by Project Grudge (the 'real' investigation) only for these purposes. Given all the above, it would not come as a surprise.
In the seventies, a TV series was based upon cases mentioned in the Blue Book Report.