I recently had to learn BGP in order to fully understand what I was doing when migrating from our Linux based Zebra routers (crappy and unstable) to our nice shiny new Juniper routers.
We had a couple of books at work, but by far the best was BGP: Building Reliable Networks with the Border Gateway Protocol. It hasn’t changed much since it was released in 2003, but neither has BGP so that really isn’t an issue, and with the recent BGP issues that keep occuring, a good knowledge of it is pretty important to any network admins out there, especially those like me with their own AS numbers!