Search for good general linux routing docs

I am thinking about setting up a server of mine as a router, with existing services still running, and set it up directly on the server instead of a VM, along with some vpn shenigans. I found this and I plan to read it: Linux Advanced Routing & Traffic Control HOWTO, but is that a good resource? I worry because it appears somewhat old, is it still good to refer to? I believe we are still using iproute2 right now. Are there better resources to refer to? I also found this, but this is more of a guide than general docs: Home Network Part 1 - DIY Home Router with NixOS - JJP Software

Having gone through setting up a DIY NixOS router(including vpn shenanigans!) I would heavily recommend grokking nftables in addition to the routing stuff.