I downloaded the client-based version of SQL Server 2008 R2 Books Online, however the installer wanted to upgrade my existing version of Books Online. Rather than do that, I'll just be using the MSDN version of SQL Server 2008 R2 Books Online which you can find here.  First thing I always look for in a new BOL is the What's New section - which if you're viewing on MSDN, is found under the following branch: "SQL Server\SQL Server 2008 R2\Product Documentation\SQL Server 2008 R2 Books Online\Getting Started\What's New".  Then I immediately jump to "What's New (Database Engine)" because that's what pays the bills for me.  You'll find for the that "Scalability and Performance Enhancements" and "Manageability Enhancements" have R2 feature additions - so be sure to give those sections a read.