[Nothing]

Go to Library Genesis or order them/find at a local library idc and READ:

Computer Networking: A Top Down Approach (really helped me learn computer networking)
Operating Systems: Three Easy Pieces (helped me with OS learning)
Absolute OpenBSD and Book of PF (OpenBSD books. OpenBSD is the best OS to date but sadly still doesn't support much popular software)

I have other recommendations, but since this is a computery forum ill keep it at that