W3C Document Object Model (DOM) Level 1 Specification (Second Edition)

PDF:  http://www.w3.org/TR/2000/WD-DOM-Level-1-20000929/DOM.pdf

Plain text:  http://www.w3.org/TR/2000/WD-DOM-Level-1-20000929/DOM.txt

A basic reference on the DOM spec.

DOM Tutorial

HTML:  http://www.w3schools.com/dom/default.asp

A good introduction to DOM maintained by W3Schools.