
https://www.amazon.co.uk/dp/B0015V9DQ0/
It's a wee bit out of date having been written ~20 years ago, but it gives you the general gist of how BGP works as pertaining to the actual internet rather than the RFC theory.
I used it (and the first edition) when I got dropped into a project to build out an internet exchange point with a route collector and all that good stuff circa ~1999.
If you really want to understand BGP, read https://www.amazon.com/Internet-Routing-Architectures-2nd-Ha...
or, if you’re looking for something online: http://www.tcpipguide.com/free/t_TCPIPBorderGatewayProtocolB...
https://www.amazon.co.uk/dp/157870233X/
It's likely a bit out of date these days, but it's one of the best books on BGP I'd read back in the day when I was involved with building an internet exchange point circa 1999.
As far as BGP goes, Halabi's _Internet Routing Architectures_ [0] is pretty much considered the "bible". It's really old nowadays but it covers BGP4 (the current version in use) and not much has really changed.
I'm sure some of the newer BGP books are excellent as well but I can't personally recommend them as IRA and the (old) CCNP BGP book are all I've ever read/used (while preparing for the CCNP certification and in my day job).
Of course, pretty much everything is covered in RFC 4271 [1] (and updates) although the RFCs can be a bit "dry".
It was my goto book when I worked on BGP-4 odds and sods.