If you have started with Drupal and want to expand your knowledge, you either need to sit and spend time on Drupal Handbooks or buy some Drupal books.
Here I am giving some good collection of PDFs available on the web, which can be used for beginner or intermediate developers.
- Cheat sheet for Themes
- Drupal module developers reference
- Building next generation Public Library websites with Drupal
- Drupal Handbook - A complete reference for those interested in Drupal, both novice and experienced Drupal administrators, Drupal users and Drupal developers.
- How to effectively manage and release your drupal contributions
- Developing for Drupal
- Form API 2.0 Cheatsheet
All these books are in PDF format so you will require Adobe Acrobat to view them.
This list of resources were found at mydrupal.com.