I've met two main kinds of designer. The kind I don't get on with too well is someone who wants to create works of art, and has the closest paying job they can get to the one they want. There's nothing wrong with making things look nice, of course, unless doing so causes you to tread on the toes of the second, and in my opinion, preferable kind of designer.
The kind of designer I like is the kind who makes things easy to use. At first, this doesn't sound as interesting as making things look nice, but it's actually a fun and fulfilling challenge. Just stop what you're doing for a second and actually think about the interface of something, anything from your chair or desk to your computer or web browser, and consciously think about how you're supposed to interact with it. Someone had to decide what it's made of, how it works, and where to put all its features. Their job was to make it as simple and intuitive as possible, so you don't even need to think about interacting with the object, you just simply do it.
Don't Make Me Think is a book about making your websites as simple and intuitive as possible. I firmly believe it should be read by everyone involved with the creation of websites, from web developers such as myself who often have a too machine-centric approach to interface design instead of a user-centric one, all the way to designers who started off creating print work and haven't yet mastered the subtleties of the web medium. Yes, there's a big difference.
I don't want to give away all the advice in the book, but most of it focuses on how to make everything obvious to the user at a glance -- what the site does, which section she's in, and what she should click on next to get to where she wants to be. When building, say, an online shop, I cannot stress the importance of these things enough. They can make the difference between a user buying something from your shop or feeling so discouraged that she'll go elsewhere, to a rival site that does follow these principles.
If you make websites for a living, there are a few books that should be on your shelf. Most of these will be references for the languages, libraries and framework you use. The other one should be a guide to making sites easy to use. This is the only such guide I've seen, and it's good enough that I won't be worried about missing out on better advice for a few more years, when it inevitably starts to show its age. Read it, and for your users' sakes, implement these principles the next time you design a site.