SharePoint Admin Guides / Documentation

SharePoint Team Services 2001 Product Documentation
http://technet.microsoft.com/en-us/library/dd349940.aspx

Windows SharePoint Services 2.0 Administrator's Guide
http://www.microsoft.com/downloads/details.aspx?FamilyID=A637EFF6-8224-4B19-A6A4-3E33FA13D230&displaylang=en

SharePoint Portal Server 2003 Administrator's Guide
http://www.microsoft.com/downloads/details.aspx?familyid=47405143-F586-4941-92F5-048BF00CF332&displaylang=en

Evaluation for Windows SharePoint Services 3.0 technology
http://technet.microsoft.com/en-us/library/cc287617.aspx

Microsoft Office SharePoint Server 2007
http://technet.microsoft.com/hi-in/office/sharepointserver/default(en-us).aspx

Microsoft Search Server 2008
http://technet.microsoft.com/hi-in/enterprisesearch/ee670396(en-us).aspx

Microsoft Office Project Server 2007
http://technet.microsoft.com/hi-in/office/projectserver/default(en-us).aspx

Forefront Security for SharePoint
http://technet.microsoft.com/hi-in/forefront/serversecurity/bb734828(en-us).aspx

For SharePoint 2010 right now I recommend
Microsoft SharePoint Team Blog
The official blog of the Microsoft SharePoint Product Group
http://blogs.msdn.com/sharepoint/archive/2009/05/07/announcing-sharepoint-server-2010-preliminary-system-requirements.aspx