If you have started with Drupal [1] 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.
All these books are in PDF format so you will require Adobe Acrobat [9] to view them.
This list of resources were found on mydrupal.com [10].
Links:
[1] http://drupal.org/
[2] http://www.minezone.org/blog/wp-content/uploads/2007/12/drupal-theme-developers-cheat-sheet.pdf
[3] http://groups.drupal.org/files/mod_dev_pres.pdf
[4] http://www.blyberg.net/downloads/Drupal-ALA07.pdf
[5] http://www.24ix.de/fileadmin/documents/Drupal/Drupal.pdf
[6] http://dwwright.net/release-talk/release-handout_v1.pdf
[7] http://www.rhinocerus.net/files/Developing_for_Drupal.pdf
[8] http://www.lullabot.com/files/formapi.pdf
[9] http://www.adobe.com/products/acrobat/readstep2.html
[10] http://mydrupal.com/drupal_white_papers_and_free_stuff
[11] http://duvien.com/muslim-and-ilses-wedding
[12] http://duvien.com/new-drupal-book-learning-drupal-6-module-development-matt-butcher