On the same subject I've found very useful "The Little Manual of API Design" by Trolltech (the makers of Qt): http://chaos.troll.no/~shausman/api-design/api-design.pdf

The documents start stating some characteristics of Good APIs:

  1. Easy to learn and memorize
  2. Leads to readable code
  3. Hard to misuse
  4. Easy to extend
  5. Complete