This is probably the best best tip I'm going to share so far with regards to Sharepoint.
Think of Sharepoint as a pre-built framework on top of ASP.NET, if you are working within the confines of the multitude of options Sharepoint gives you, then you need a big thick book on Sharepoint.
If you are thinking about customizing Sharepoint, then you're beginning to face a problem I had.
How does it all work, and perhaps more importantly, why does it just not work?
You can resort to Google. In which case, I hope you find this blog.
Or you can grab Reflector and peek inside the secrets of Microsoft.Sharepoint (WSS) and Microsoft.Sharepoint.Publishing (MOSS) assemblies.
I'll share with some findings real soon.