in reply to
On Interfaces and APIs
Although many of Jakob Nielsen's observations are sound, I'm not a fan of his recent writing -- I stopped reading AlertBox a few years ago when too many of his postings seem to be of the 'here's some cursory information -- buy the whitepaper if you want more'.
If you want a quick, simple book on usability concepts for web design, take a look at Steve Krug's Don't Make Me Think. It's the first book I typically give people on usability, as it's a very usable book itself.