There has been a lot of confusion lately because the TFS SDK was erroneously removed from the VS SDK.  We are working to get it put back into the VS 2008 VS SDK.  Until then, you can use the VS 2005 SDK - which still contains the TFS SDK docs.  There were very few object model API changes in TFS 2008 so the TFS 2005 docs are 99% accurate.

http://msdn2.microsoft.com/en-us/library/bb130146(VS.80).aspx

Brian