One of my highest visited posts is the one I wrote about usability. I've since added updates to direct people to other ideas. Sometimes I feel like I should write more on this topic because there is a large void on it within the agile community, and I spent more than five years focused on it. But the reality is that I have been focused on process, PM, agile for the last 4 years now. Usability is like SEO, you need to be living it today to be an authority. My history in it allows me to carry good conversations, but I'm not going to pretend I'm still an expert.
With that lengthy, wordy introduction... here is a good second part about usability in agile. I just happen to not be the one who wrote it.