Usability includes things like - does the layout make sense? can the user find/do what they set out to do without frustration? does the user understand what you/your site is about? It is not as easy to accomplish as you would think. Personal bias and preference can come into play. Not to mention that you know your site and you may assume that all you know of it and its purpose is obvious to everyone when in truth, it isn't that obvious.
This book is designed to help with the usability part of this. It is a very accessible, easy to read "common sense approach" to evaluating and creating a better website for your visitors. Krug goes through his talking points in a way that is easy to understand and with plenty of visual examples. There were quite a few times where I thought to myself that "well, yeah - that makes sense, I knew that" but it is always good to have these kind of reminders when you are muddling through a huge project like web site design or redesign. This is a quick read that will help you inform your process when tackling this kind of project from a known expert on this topic.
Don't Make Me Think, Revisited
by Steve Krug
New Riders Publishing, 2013