The Book
Cracking Drupal: A Drop in the Bucket
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.
Security Review Service
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, Growing Venture Solutions offers a Security Review Service for Drupal Sites.