DESCANSO book series
When researching technologies for communicating across long distances in space, books are invaluable. But free books are better!
I’ve found the DESCANSO book series from JPL to be invaluable. Hard copies are available, but if you’re happy reading PDFs you can download them right off the JPL site. These titles cover everything from bandwidth-efficient digital modulation to antenna arraying techniques, optical communications and software defined radio.