Also check the CCNA course "Jeremy's IT Lab" on YouTube. Highly recommend.
I think in general more of the documentation and information is in blog posts and RFCs, so I’d start there, e.g. [2] is quite nice.
1: BGP (Border Gateway Protocol): from theory to practice - https://amzn.eu/d/i6EUYGj
2: https://labs.ripe.net/author/samir_jafferali/build-your-own-...