Steve Krug's Don't Make Me Think is my favorite book on Web site usability, and he has just published a completely updated version. It's arguably the Elements of Style for Web planning.
If you're too busy to read the book, or the rest of this post, the book's title is great advice in/of itself. Don't make the site visitor have to approach your site like a puzzle or mystery... spoil the ending and get them to the right page quickly.
Since the first edition of DMMT came out waaaay back in 2000, Web designers and developers have relied on Steve’s guide to help them understand intuitive navigation and information design. Now he's back with fresh perspective and updated examples and a new chapter on mobile usability.