The almost 500 page Upgrade Technical Reference guide for SQL Server 2008 R2 is available at the dedicated upgrade site. This guide not only covers upgrading from versions prior to 2008, it also covers considerations (minimal) for upgrades between SQL Server 2008 and 2008 R2.
If you are looking for reasons to upgrade to SQL Server 2008 R2, you can find these in the Why Upgrade Datasheet.
Following are the key topics covered: