IBM had two great teams totalling 17 skilled resources working on Portal 6 redbooks. I know many of the people that worked on these drafts and the feedback they have provided me is that they really took the time to cover every detail to help accelerate a migration and plan for a large scale Portal implementation.
WebSphere Portal 6 - Best Practices for Enterprise Scale Deployment
Best Practices for Migrating to IBM WebSphere Portal 6
IBM Premium Service Manager/IT Specialist
Lotus, Portal, and Collaboration Software
Two must read Redbooks for Portal 6