There is a great chapter on BGP by Hari Balakrishnan. I believe the chapter is available on MIT Open Courseware. It should be under the Computer Networks class.

Edit: found it. See Lecture 4: http://ocw.mit.edu/courses/electrical-engineering-and-comput...

Google has a couple good starting points: http://www.enterprisenetworkingplanet.com/netsp/article.php/... http://blog.pluralsight.com/bgp-border-gateway-protocol