Recommended Study Resources
Use the CCNP VPN 642-648 Official Certification Guide. Like mainly all Cisco Press Certification Guides, besides explaining the technologies needed for both the exam and real life practice, this book offers many other tools to help with exam preparation, including memory tables, notations of the most important topics in each chapter, and a CD question bank with exam software.
The CCNP VPN 642-648 Quick Reference Guide provides you with a very good overview of technologies you need to know for the exam and not only. Every chapter briefly explains each technology and gives you a quick look into configuration aspects from both GUI, handled by ASDM software for ASA, and CLI perspective. Book can be useful as a summary after reading the Official Certification Guide or as a refresher if you are already familiar with most topics.
If not certain what specific commands achieve, or what options you have on configuring it, check the free, online Command Reference Guide for the ASA.
As starting with versions 8.3, 8.4 many new features became available, you might want to check the Cisco ASA New Features by Release document.