VPNs and NAT for Cisco Networks (Cisco CCIE Routing and Switching v5.0 Book 3)
Designed for the CCIE v5, but suitable for any ability - from CCNA to CCIE.
Part one of this book will walk you through building a simple GRE tunnel into an IPSec secured, optimized and scalable DMVPN network.
Part two expands the network to include Network Address Translation, as well as IPv6 transition mechanisms.