One book I highly recommend for learning about how to design a usable website is Don't Make Me Think by Steve Krug. This book should be your starting point if your job depends on you understanding the fundamentals of maintaining or designing a functional website. It takes about two to three hours to read, and each page has useful insights.
One observation that he makes early on is that when people are hurried (as are most visitors to your website) they don't look at multiple choices and then pick the most optimal choice. They scan for the first solution that appears to be acceptable. Once they find an acceptable solution, they test it out. If it works, they're happy. If it doesn't, they're not. This means that most people "muddle" their way through a website by scanning pages.
This is why most well-designed websites organize their information in clear, segmented chunks that are easily scannable. When you're designing a website, you should always strive to optimize it for the way people will actually use it, not the way you hope they will use it. And if your job in any way depends on designing or improving a website, reading Steve Krug's book will be time well spent.