webpart how and checklist

Microsoft articles: "Checklist for Testing SharePoint Web Parts"

If you're new to web parts start here The ultimate webpart "hello world" article

Useful microsoft presentations

Oodles of sharepoint presentations from some Microsoft conference or other, my personal favourite is Nathan Fink's which deals with the object model.

code security and web parts

Jan Tielman's blog has an excellent set of work-rounds for getting you web parts work.

The long way round is described by Microsoft "Microsoft windows sharepoint services and code access security"

For ASP.NET you could also try looking at "Using code access security with ASP.NET" patterns and practices

member web part work round

useful looking blog

Branding Sharepoint with templates

Several long, in depth Microsoft articles describing all anyone could need about templates.