Doyle masterfully explains the paradigm shift. In Volume I (OSPF/EIGRP), you trust everyone. In Volume II (BGP), you trust no one. The book breaks down Autonomous Systems (ASs) and why the internet is a federation of warring tribes rather than a single country.
One of the most valuable sections is the multi-homing case study. Should you get a default route? A full table? Partial tables? Volume II quantifies the CPU and memory cost of a full internet table (roughly ~900k routes today, though the book used older numbers) versus the control granularity. It teaches you how to your way to a perfect ISP handoff. Routing TCP IP- Volume II -CCIE Professional Development