Probably one of the best books in terms of explaining SDR / DSP fundamental concepts in an easy to understand way.
"PySDR: A Guide to SDR and DSP using Python" free book
"....“Revolution” is a term thrown about with a lot less care than it probably should be, especially in fields like electronics. It’s understandable, though — the changes to society that have resulted from the “Transistor Revolution” or the “PC Revolution”......."
".... Such is the case with software-defined radio (SDR) and digital signal processing (DSP). These two related fields may not seem as transformative as some of the other electronic revolutions, but when you think about it, they really have transformed the world of radio communications....."
PySDR: A Guide to SDR and DSP using Python
I recommend (and fully agree with the quoted statements from Hackaday)