Once you're past the fundamentals, if find yourself interested in high-performance networking, I recommend looking into userspace networking and NIC device drivers. The Intel 82599ES has a freely available (and readable!) data sheet, DPDK has a great book, fd.io has published absolutely insane benchmarks, ixy [1] has a wonderful paper and repo. It's a great way to go beyond the basics of networking and CPU performance. It's even more approachable today with XDP – you don't need to write device-specific code.
[1] https://github.com/emmericp/ixy