DEVELOPMENT OF THE SINGLE ROTOR HELICOPTER
Written by one of the first engineers working with Igor Sikorsky on the development of his brain child (single rotor helicopter). The one point of some critic is the enginering and technical description, that is even a bit too much for one with design experience in aircraft engineering.
Gives a good account of the early development of the helicopter, the problems involved at a time that practical war planes had a much higher priority (1940-45 time span). Good book with a lot of technical details, but requires some degree in engineering to read.
Kind regards.