Linq to XML

http://www.hookedonlinq.com/LINQtoXML5MinuteOverview.ashx

An external storage API is available for WSS 3.0

Store SharePoint data in external device other than SQL Server
Follow this link to know more http://support.microsoft.com/kb/938499/

Visual Studio extensions for SharePoint – Developer User Guide

Microsoft has released User Guide, Samples and Walkthroughs for the Visual Studio 2005 extensions for Windows SharePoint Services 3.0, v1.1.

The user guide is downloadable from here

Planning and architecture for Windows SharePoint Services 3.0 technology

If you want to know more about planning and architecture for WSS 3.0, please visit the following link.

http://technet.microsoft.com/en-us/library/cc288426(TechNet.10).aspx

In this you can learn about,
Determine organization and user needs (Windows SharePoint Services)
Planning worksheets for Windows SharePoint Services 3.0
Plan Web site structure and publishing (Windows SharePoint Services)
Plan for content and search (Windows SharePoint Services)
Plan site and content security (Windows SharePoint Services)
Plan communication (Windows SharePoint Services)
Plan for site creation and maintenance (Windows SharePoint Services)
Plan for system requirements (Windows SharePoint Services)
Design server farms and topologies (Windows SharePoint Services)
Design logical architecture (Windows SharePoint Services)
Plan for authentication (Windows SharePoint Services)
Plan for and design security (Windows SharePoint Services)
Plan for performance and capacity (Windows SharePoint Services)
Planning and architecture for Windows SharePoint Services 3.0 technology summary
Downloadable book: Planning an Extranet Environment for Windows SharePoint Services
Plan for data protection and recovery (Windows SharePoint Services)

You can also download this from the below mentioned Url
Downloadable book: Planning and architecture for Windows SharePoint Services 3.0 technology