Ok, another Monday freebie - Qualys has a free e-book PCI Compliance for Dummies available athttp://www.qualys.com/docs/PCI-for-Dummies.pdf
At 68 pages, it provides a really good introduction to the topic.
For those that want a comprehensive reference on the topic, PCI Compliance: Understand and Implement Effective PCI Data Security Standard Compliance by Chuvakin and Williams is worth the investment.