Drupal Commerce is constantly evolving, but we're working hard to keep this specification current so it expresses in plain text how the various pieces of Commerce have been structured and how they work together. This handbook is good for getting a big picture understanding of how we have developed Drupal Commerce and should be a handy reference for writing your own modules to integrate with the core Commerce modules.
Currently the Info hooks and API utilization guides contain the only complete content. The specification is not a tutorial or guide for new users, nor is it a complete set of API documentation. We'll post links here to more appropriate resources for those things as they develop.