Plan authentication methods (Office SharePoint Server)
http://technet.microsoft.com/en-us/library/cc262350.aspx
Design extranet farm topology (Office SharePoint Server)
http://technet.microsoft.com/en-us/library/cc263513.aspx
Downloadable book: Planning an Extranet Environment for Office SharePoint Server
http://technet.microsoft.com/en-us/library/cc262400.aspx
Planning, Designing; Securing an Extranet and Internet Facing WSSv3 and SharePoint Server 2007 Environments
External Collaboration Toolkit for SharePoint