Don't Make Me Think
Great, quick read on usability. Sort of a classic, really. Much of it might seem obvious, but then that's the point!
From the publisher: "The title of the book is its chief personal design premise. All of the tips, techniques, and examples presented revolve around users being able to surf merrily through a well-designed site with minimal cognitive strain. Readers will quickly come to agree with many of the book's assumptions, such as "We don't read pages--we scan them" and "We don't figure out how things work--we muddle through." Coming to grips with such hard facts sets the stage for Web design that then produces topnotch sites."