If you really want to understand the heart of SDR I suggest starting with Dan Tayloe's article
Just perusing the articles you can tell what is important in the SDR arena. Things like oscillator jitter and low low noise amplifiers
The other thing that is most important is to understand that SDR is done with DSP digital signal processing. This means the processing is going on in virtual time, not real time. The simplest analogue radio is a crystal radio aka a diode detector. A signal hits the antenna, hits the diode and audio is immediately produced. This is real time. In a commercial ham receiver you add amplifiers and detectors and filters and blankers into the signal path stretches things out a little bit between antenna and ear, but the signal is processed in virtual time with the SDR and that means you can manipulate it much more dramatically (aka mathematically) with far fewer components.
Much good stuff to consider on these pages and thanks to Jim for pointing this site out.