For best SharePoint Development practice, please follow these ( as per Microsoft recommendations):

• Development Tools and Techniques for Working with Code in Windows SharePoint Services 3.0 (http://go.microsoft.com/fwlink/?LinkId=123413)
• Working with ASP.Net 2.0 Web parts and WSS 3.0 (http://go.microsoft.com/fwlink/?LinkId=123429)
• Best Practices: Common Coding Issues When Using the SharePoint Object Model (http://go.microsoft.com/fwlink/?LinkId=123414)
• Windows SharePoint Services Developer Center (http://go.microsoft.com/fwlink/?LinkId=123428)
• MSDN Webcast: Essentials of Application Security (Part 1 of 3): Secure Communications (Level 200) (http://go.microsoft.com/fwlink/?LinkId=123430)
• SharePoint Server 2007 Developer Portal (http://go.microsoft.com/fwlink/?LinkID=119449)
• Microsoft Anti-Cross Site Scripting Library V1.5 (http://go.microsoft.com/fwlink/?LinkId=123431)
• Improving Web Application Security: Threats and Countermeasures (http://go.microsoft.com/fwlink/?LinkId=123432)
• Exception Management Architecture Guide (http://go.microsoft.com/fwlink/?LinkId=123433)
• SharePoint Products and Technologies White Paper: Implementing Microsoft Office SharePoint Server 2007 and Windows SharePoint Services 3.0 Solutions (http://go.microsoft.com/fwlink/?LinkId=114834)
• How to programmatically test for canonicalization issues with ASP.NET (http://go.microsoft.com/fwlink/?LinkId=123434)
• Validating ASP.NET server controls (http://go.microsoft.com/fwlink/?LinkId=123435)
• MSDN Webcast: Digging into Server-Side State Management (http://go.microsoft.com/fwlink/?LinkId=123436)
• SharePoint Products and Technologies customization policy (http://go.microsoft.com/fwlink/?linkid=92311)
• SharePoint Products and Technologies Customization Best Practices (http://go.microsoft.com/fwlink/?LinkID=123984)

Also WSS and MOSS SDK are always helpful.