Scott Hanselman recently interviewed Tom and Mary Poppendieck on his Hanselminutes podcast, and then there is the recent post Agile Versus Lean by Martin Fowler. Check them both out. Now.
Even if you've already studied Scrum and XP, you might want to pick up a book or two on this particular strain of agile development methods. It can help you clarify and solidify some of the concepts you're already working with and potentially give you some new tools and techniques to optimize your current methodology.