Well, if 40 pages specific to the Pi isn't detailed enough, you may need to get some more background and I can't even think of how that might be done.
Python isn't a toy language just because it's been chosen to teach students. If you've been misinformed that it's just a pokey scripting language because it's interpreted, you should be aware that Python is routinely compiled into modules (as .pyc files) once functionality has been worked out and known bugs have been eliminated. Python is generally implemented in C under the hood, with a bit of assembly language in critical sections if needed, so it's no slouch.
The algorithms for I/O on the Pi are independent of the implementation language, so the C code to access the GPIO ports is analogous to the equivalent Python. As for Ethernet and USB, pretty much any ARM Debian-compatible code (and most ARM Linux libraries, for that matter) should work since both communications methods use hardware/OS-independent standards. It's all about configuring and opening a port for reading or writing, stuffing the appropriate source and destination address/port and content data into packet header and body data structures, blasting everything on its way, verifying that it was successful, and lathering, rinsing, and repeating until all of the data has successfully made it to its intended destination.
The best things in life aren't things ... but, a Pi comes pretty darned close!
"Education is not the filling of a pail, but the lighting of a fire." -- W.B. Yeats
In theory, theory & practice are the same - in practice, they aren't!!!