some resources i find interesting and helpful learning low level system stuff: https://wiki.osdev.org/Expanded_Main_Page http://www.osdever.net/

That and documentation for the platform you are working on, in my case intel/amd manuals but not sure what system you are working on, i presume one of those though.

it's also interesting to look into assembly/ assemblers, disassemblers, perhaps look at capstone / keystone frameworks to learn about that. linkers & loaders is also interesting.

it really depends on where you want to go with low level/ system things what is relevant though, as a lot of it involves platform specific details.