I found "BGP4: Inter-domain Routing in the Internet" by John Stewart III [0] to be very approachable. It's likewise ancient by tech world standards (published in 1998!) but that's because, amazingly, BGP has not fundamentally changed in all that time. It's cheap and less than 200 pages; definitely recommend it as a primer.

[0]: https://www.amazon.com/dp/0201379511/