Modules

• Creating modules - a tutorial: Drupal 6.x
• Creating Drupal 6.x modules
• Hooks
• Converting 5.x modules to 6.x
• Documentations for core modules

Menu system

• Drupal menu system
• Menu system - Drupal 6 APIs
• Converting 6.0, 6.1 modules to 6.2

Forms

• Forms API
  • Forms API Quickstart Guide (forms_api.html)
  • Drupal 5.x to 6.x FormAPI changes
  • Forms API Reference
  • Form builder functions
  • Form generation
  • Create a module configuration (settings) page
  • Input validation
  • Code snippet: How to set the disabled attribute of a CCK field
• Batch processing
  • Batch operations
  • Example batch definitions
• Files
  • File interface

Node

• Create new content-type for Drupal 6.x
• Node module
• Node access rights

Themes

• Theme guide
• Converting 5.x themes to 6.x
• Using the theme layer for modules
• Formatting
• Default theme implementations
• Sub-themes, their structure and inheritance
• Discussion: Cannot display custom field

Database

• Schema API
• Database abstraction layer
• Tips:
  • How to connect to multiple databases within Drupal
  • How to write database independent code
  • How to update a module's weight
  • Updating tables: don't use hook_schema()

Javascript

• Javascript, jQuery and AJAX 
• JavaScript Startup Guide

Images

• Image toolkits

Searches

• Search interface

Design Pattern

• Drupal Programming from an Object-Oriented Perspective