The primary source of documentation is on Read the Docs (https://hackrf.readthedocs.io/) or in the docs folder in the HackRF repository on GitHub (https://github.com/mossmann/hackrf/tree/master/docs).

This directory contains supplemental documentation.

(photo jawbreaker-fd0-145436.jpeg by fd0 from https://github.com/fd0/jawbreaker-pictures)