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 intermidiate developers.

1. Cheat sheet for Themes.
2. Drupal module developers reference.
3. Building next generation Public Library websites with Drupal
4. Drupal handbook.
   A complete reference for those interested in Drupal, both novice and experienced Drupal administrators, Drupal users and Drupal developers.
5. How to effectively manage and release your drupal contributions
6. Developing for Drupal
7. Form API 2.0 Cheatsheet
8. Drupal Multi Site Management Tools
9. How to Install Drupal on a Windows Workstation
10. Drupal CVS Cheatsheet