Phase noise in oscillators was investigated in the mid 1960s by Dave Leeson and he came up with his classic phase noise equation some time around 1966 I believe. This allowed the noise of an oscillator to be modelled wrt flicker noise at 9dB/octave and thermal noise at 6dB/octave and the noise floor was a function of the loaded Q and frequency etc.
I think he was asked to do this work because the phenomenon of reciprocal mixing had recently been appreciated in those days although I don't know when it was first called reciprocal mixing. But clearly there was some demand for this research work to be completed (in a hurry) back in those days.
I've used Leeson's equation numerous times when designing oscillators from HF through to many GHz and it always seems to give good results. The only thing I struggle to predict accurately up at many GHz is the flicker corner frequency. But I usually model the far out phase noise at 100kHz and 1MHz offsets within 3dB of the 'real circuit when tested, even for oscillators up at several GHz
I also have a copy of the "White Noise Book" here and I've done a fair bit of NPR testing at work although this was 20 years ago...