This book is ancient, but still 100% useful. I read it like 4 or 5 times almost 20 years ago, and think of it often https://www.amazon.com/BGP-Building-Reliable-Networks-Protocol-ebook/dp/B0043D2F30/ref=sr\_1\_1?crid=1QDMM8QXK73I1&keywords=bgp&qid=1645748240&sprefix=bgp%2Caps%2C248&sr=8-1