The 1950s record is dominated by the Air Force investigative arc — Sign giving way to Grudge, Grudge to Blue Book — and by the CIA's institutional response in the form of the Robertson Panel. The panel's recommendation that the public 'reaction' to UFOs be managed through education and counter-amateur-investigation measures shaped US government UFO communication for the next four decades.
Notable non-US material includes the September 1957 RB-47 incident (electromagnetic-interaction case over the southern United States), the 1955 Kelly–Hopkinsville encounter in Kentucky, the 1957 New Mexico Sandia–Kirtland sightings, and the contemporaneous European reporting collected by the French Gendarmerie that would later become GEIPAN's pre-1977 archive.
Why this matters. The 1950s is when the institutional posture solidifies. The 'unknown but unimportant' framing that Robertson recommended became the operational default and remained so until the 2017 NYT story.