"Cracking Drupal
is probably going to be
the first Drupal book I buy."
- Angie 'webchick' Byron
Chris Shattuck wrote a nice review that includes various bits of advice for improvement:
http://chrisshattuck.com/blog/book-review-cracking-drupal-greg-knaddison
Most notable
* Not enough coverage of form tokens and Ajax
* More advice on SSL and mixed-mode SSL
* Needs more humor (maybe)
Written by a Drupal expert, this is the first book to reveal the vulnerabilities and security issues that exist in the sites that have been built with Drupal and how to prevent them from continuing.
Curious about Drupal's security? Consider Drupal but want to make sure it's good enough? Read the Drupal Security Report
If you are concerned about whether your site is secure, consider using the Security Review Module to get an initial sense. If you're looking for something much more in depth than a module or a book, Drupal Scout offers a Security Review Service for Drupal Sites.