The one thing I noticed when I was working on my CCNP is the big difference between the Cisco Press Books that were the "Exam Certification Guide" and the "Self-Study" books. The Self Study books matched up with the official courseware, and I thought gave a better explanation of topics. The Exam Cert books seemed like they expected you to have a pretty good foundation, and went into a little more detail with less background.
I also used a lot of documentation off of Cisco's website. Using the blue print for each exam, I'd do searches on the website, and get additional information on each topic. Some really good docs out there on BGP and Spanning-tree.