Here's a book that must be on every ScrumMaster's book shelf:
Agile Retrospectives shows you how to best facilitate this essential practice and to keep it fresh by choosing from many different types of retrospectives. It offers great advice on eliciting feedback from everyone and digging deep into underlying issues.
Retrospectives will be the topic of my IGDA Leadership Forum 2010 session. We'll be talking about the best flow for retrospectives and running a few of them to discuss how well the Leadership Forum functioned.