Many resources are available for the Drupal developer. The most useful of these are listed here.
Some Drupal code resources follow.
Access to the CVS tree containing the Drupal core codebase and contributions repository has been covered in Chapter 21; however, a convenient web interface for browsing the repositories is available at the preceding URL. Especially nice is the ability to do color-coded diffs quickly.
The comments from Drupal functions, as well as the documentation available in the
contributions/docs/developerarea of the Drupal contributions CVS repository, are available at http://api.drupal.org. Code is searchable, cross-referenced, ...