the Lotus Notes and Domino 6 Programming Bible by Brian Benz and Rocky Oliver. It is an EXCELLENT resource. You can order it online through Amazon, or Brian’s site http://www.brianbenz.com/, or Rocky’s site http://www.lotusgeek.com/
Read this forum regularly. The questions / answers / discussions are extremely helpful in “getting” how Notes works.
Read the blogs. Do a Technorati search for “SNTT” or “Show and Tell Thursdays” -there are tons of helpful tips / ticks out there.
(This is IMO the most important) Take a training class (or 2, or 3) from an authorized training center. I cannot stress enough how important training is. Consider not only development, but administration as well. The two reinforce one another. As an extension to this, look into getting certified as well. The courses and tests required for certification will give you a solid understanding of Notes.
I’d like to throw my two cents in here: I’d move #4 up to #1. Or at least after #1 so you can possibly take the course while you wait on your book to arrive. Then do the obvious thing, and buy a javascript book and a book on DHTML and AJAX. “Ajax in Action” is an excellent resource… you can learn a lot about javascript from it in addition to the AJAX techniques. Also, as you didn’t mention your previous experience… if you’re not already a VB developer, start learning JAVA. Actually, if you are, start learning JAVA anyway. Not required for web development, per se. I do all kinds of web apps in Domino with javascript and LS agents to handle the back-end data. However, knowing JAVA gives you room to branch out in the IBM world and get into servlets, etc.