From Scott Ambler:

One of the great debates raging within the IT industry is whether or not agile software development techniques work.  My experience, and the experience of thousands of others, is that they do.  One of several reasons why agile techniques are so effective, in my opinion, is that they reduce the feedback cycle between the generation of an idea (perhaps a requirement or a design strategy) and the realization of that idea.  This not only minimizes the risk of misunderstanding, it also reduces the cost of addressing any mistakes.  In this article I explore this idea in detail. 

Very detailed analysis of how usability can be a key component of agile methodologies. Explores the misconceptions and challenges of both schools with respect to the other. Details the agile development methodology. Contrasts the philosophies of both agile and UEX approaches. Discusses user-experience modeling in an agile framework with great examples and explanations.

Deep analysis of the tasks that must happen in an iteration - development, analysis, testing, etc.  Focus on the problems with staggering them too much, and suggestions on how to properly organize the team around those tasks.