Patrick Tisseghem has just written a couple of articles for MSDN on building Windows SharePoint Services solution with Visual Studio 2005 Extensions. Well a couple of articles is putting it lightly it's 58 pages of printed material that is more than a starting point it's everything you need to know to build solutions on WSS.
Part 1: http://msdn2.microsoft.com/en-us/library/bb530302.aspx
Part 2: http://msdn2.microsoft.com/en-us/library/bb530301.aspx