There was excellent docuent on Cisco (better than book). I can search
it, if you want.
This one is not too bad ..
Practical BGP (Russ White, Danny McPherson, Srihari Sangli)
I have two books that I'm quite fond of:
BGP4: Interdomain Routing in the Internet, by John W. Stewart
Internet Routing Architectures, 2nd Edition, by Sam Halabi
I found both books to be extremely helpful.