IPv6 and IPv4 are covered in more detail in [STAL04]. [CHEN98] provides a good discussion of an IPSec design. [FRAN01] and [DORA99] are more comprehensive treatments of IPSec.
CHEN98 Cheng, P., et al. "A Security Architecture for the Internet Protocol." IBM Systems Journal, Number 1, 1998.
DORA03 Doraswamy, N., and Harkins, D. IPSec. Upper Saddle River, NJ: Prentice Hall, 2003.
FRAN01 Frankel, S. Demystifying the IPSec Puzzle. Boston: Artech House, 2001.
STAL04 Stallings, W. Computer Networking with Internet Protocols and Technology. Upper Saddle River, NJ: Prentice Hall, 2004.
Recommended Web Site
NIST IPSEC Project: Contains papers, presentations, and reference implementations