Even better than good error messages is a careful design which prevents a problem from occurring in the first place. Either eliminate error-prone conditions or check for them and present users with a confirmation option before they commit to the action.

Designers are not users.

Progressive disclosure defers advanced or rarely used features to a secondary screen, making applications easier to learn and less error-prone.

Usability rules the web. Simply stated, if the customer can't find a product, then he or she will not buy it.

The system should always keep users informed about what is going on, through appropriate feedback within reasonable time.

...the book is a manifesto to make the Web atone for the sins of computers and regain a level of simplicity that can put humanity at peace with its tools once again.

The more users' expectations prove right, the more they will feel in control of the system and the more they will like it.