Updated version of two ADFS documents has been published on Technet pages in December.
ADFS Design Guide deals with ADFS design concepts and design process. Good starting point for understanding Web SSO solution with ADFS and different design concepts. Updated version delivers capacity planning information, web browsers compatibility matrix, some design checklist templates and more.
Second recently updated document is ADFS Deployment Guide which deals with topics related to deploying different parts of ADFS to successfully accomplish our design.
Both documents are worth reading if You are considering ADFS as SSO solution or just You want to get better overview what ADFS is and how it works. In this second case Design guide document is for You.