If not, don't fret. You can still improve your site's usability by learning the tips, tools and processes necessary for such. To start with, here are the books that, when read together, will help you get a good grip on the concept of usability.
I. General Website Usability Principles
Handbook of Usability Testing: How to Plan, Design, and Conduct Effective Tests (Jeffrey Rubin and Dana Chisnell)
Whether you're a beginner who wants to know more about usability, or a professional who needs to structure your testing process, there's something in this book for you. Within its 384 pages, Handbook of Usability Testing lists the most important tips and tricks for effective usability testing. You can supplement your readings from the book through its companion site here.
Usability Testing Essentials: Ready, Set… Test! (Carol M. Barnum)
Like Handbook of Usability Testing, this book covers the basics in an easy-to-understand manner. However, Barnum goes one step further and touches on usability for international audiences – a topic often neglected in similar books. It's worth noting that Steve Krug, author of Don't Make Me Think!, personally recommends this, so you might want to check this one out for that reason.
II. Planning Your Websites UX
The Elements of User Experience: User-Centered Design for the Web (Jesse James Garrett)
In theory, the idea of "good" UX is simple enough: Don't make users think. In practice, there are so many ways to get UX right – and just as many ways to get it wrong
Fortunately, Garrett takes the hard work out of conceptualizing UX by breaking it down into five planes: strategy, scope, structure, skeleton and surface. Garrett discusses all of these in a straightforward, yet in-depth, manner, making this book an enjoyable read. But more importantly, this book helps you keep your concepts as "clean" as possible before executing them.
III. Measuring Website Usability
Measuring the User Experience: Collecting, Analyzing, and Presenting Usability Metrics (William Albert and Thomas Tullis)
If you're looking for a comprehensive book on usability metrics, look no further than this one. It starts with an introduction to basic statistics, making it useful even for would-be usability professionals who have no background in quantitative research. Additionally, the book has extra chapters for special topics (e.g. live website date, ROI data), case studies, and practical matters related to the measurement of UX data.
Granted, this isn't as comprehensive as Measuring the User Experience. In fact, it's not meant to be a standalone material if you want to know how to measure usability. But you can quickly refer to it whenever you have a metrics-related question that needs to be answered pronto.
IV. Research on User Behaviour
Observing the User Experience: A Practitioner's Guide to User Research (Elizabeth Goodman)
There's often a gap between what you think your users want, and what they really want. Luckily, it's possible to close this gap through user research, which Goodman thoroughly discusses in this book.
The book mostly covers the different methods for research: competitive research, recruitment, interviews, focus groups, field visits, diary studies, surveys, and global and cross-cultural research. It also touches on usability testing, qualitative data analysis and other matters, though it's better to read other, more comprehensive books on these subjects.
Interviewing Users: How to Uncover Compelling Insights (Steve Portigal)
This book dismantles the idea that an interview is simply a Q&A session. If done well, interviews can help you mine valuable user information you wouldn't have acquired in any other way. The book's case studies, illustrations and other visual guides are a great help when you learn the various tools and techniques packed within its 176 pages.
V. Psychology of Website Users
100 Things Every Designer Needs to Know About People (Susan Weinschenk)
This book might as well be called "The Psychology of Web Design", although Smashing Magazine already published a book of that name. 100 Things Every Designer Needs to Know About People compiles studies on user psychology, analyzes them, and discusses the implications of each in the form of "Takeaways". Some of the findings are intuitive (e.g. people can't multitask), while others might surprise you (e.g. people read longer sentences faster, but prefer shorter ones). The studies are organized according to categories like "How People See", "How People Read", etc.
Aside from the books above, you can also read these 20 great UX blogs for designers. The books will help you grasp the basics, while the blogs will keep you updated on the latest developments in UX. Go through all of them, see which ones suit you the most, and read on.