Drupal White papers , Cheatsheets and Free books
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.
4. Drupal handbook.
A complete reference for those interested in Drupal, both novice and experienced Drupal administrators, Drupal users and Drupal developers.