Here are recommendations for each of the certification exams covered in this article:
For exam 640-503: Cisco CCNP Routing Exam Certification Guide by Clare Gough, CCIE.
For exam 640-504: Building Cisco Multilayer Switched Networks by Karen Webb, CCIE.
For exam 640-505: Cisco CCNP Remote Access Exam Certification Guide by Craig Dennis and Brian Morgan, CCIE.
For exam 640-506: Cisco Internetwork Troubleshooting by Dan Farkas, CCIE, and Laura Chappell.
For all of the exams in one shot: http://www.cisco.com/warp/public/10/wwtraining/certprog/lan/programs/ccnp.html.