In this article, I would tell you the basics of Drupal API. You will learn how to find all required API information here.

Tutorials:

Online

api.drupal.org

Offline 

• Drupal 6 API in CHM format.
  • Size: 4,1Mb.
  • Source: drupal.kollm.org.
  • Search option.
  • View in Linux:
    • chmsee 
    • kchmviewer
    • GnoCHM (navigation by links and tabs works, the upper menu doesn’t work)
    • Kchm

  

• Drupal 6 API in CHM format
  • Download for registered users.
  • Size: about 2Mb.
  • Advertisement in the header.
  • View in Linux:
    • chmsee
    • kchmviewer
    • GnoCHM (menu doesn’t work)
    • Kchm – shows empty document

  
  

Drupal 5 API (en)

• Drupal 5 API in CHM format
  • Download for registered users
  • Size: about 1Mb.
• Drupal API 4.6, 4.7 and 5 in CHM format (2.4Мб).
  • Created using htm2chm.
  • No search option.

  

Use API module to analyze installed modules

This module can be used to create documentation (which is also available at api.drupal.org) for Drupal-developers. The module  description: http://drupalcookbook.ru/module/api

Good Luck!