The 3rd and final installation in Harry Fuecks' introductory article, Introducing XUL - The 'Net's Biggest Secret: Part 3 has been published on SitePoint. Nice wrap-up with excellent links to interesting projects like MozBlog (allows blog updating in Mozilla over XML-RPC) and the Luxor XUL SWT for Eclipse that will bring XUL into that very IDE. Stay tuned (I will).
An ebook on creating XPCOM components, Creating XPCOM Components has been released. Only for C++ developers though.