For more information on Mobile IPv4 specifications, the reader is referred to the specifications that were completed by old Mobile IP working group, whose website is: http://ietf.org/html.charters/OLD/mobileip-charter.html.
Specitications are being completed by the IETF Mobility for IPv4 working group, found on the group's website http://ietf.org/html.charters/mip4-charter.html. IETF specifications on the interaction of Mobile IPv4 and VPNs can also be found on this page.
There is also an excellent book written on Mobile IP by one of the founders of Mobile IP, i.e. Jim Salomon. This book, called Mobile IP: The Internet Unplugged, provides a great overview for people who do not have an IP networking background.
For more information on Mobile IPv6 specification, the reader can look at the "Mobility for IPv6" working group website http://ietf.org/html.charters/mip6-charter.html and the "Mobile IPv6 signaling and handoff optimization" working group (mipshop) website at http://ietf.org/html.charters/mipshop-charter.html. More information on the interaction of Mobile IPv6 and IPsec architecture can also be found on the Mobile IPv6 working group website.
For more information on network mobility protocol for IPv6 networks, the reader is referred to the IETF "network mobility" (nemo) working group website at http://ietf.org/html.charters/nemo-charter.html.