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It probably has no PR as it is a dynamic page which is creating a session ID for you, or maybe Google hasn't got round to updating the toolbar fed data yet.
And to say that Amazon has no backlinks is ridiculous. They probably have more backlinks than anyone (except for MSN, Google, Yahoo...). You don't need backlinks to a specific page to get PR, to the root of the domain is fine - PR increases on your home page and then that increase filters down on the internal links, thus increasing the whole site's PR.
If you needed backlinks on specific pages, Amazon would be doomed - they just have too many.
Google probably can determine an estimated PR from the domain name, checks the relevancy of the page (more important than PR) and displays the results accordingly.
Amazon will be a recognised authority for that (anchor text backlinks).
The "Master and Commander" bit is just pulled from on-page factors, but neither of those words will have an authoritative site (unlikely anyway). Google ignores the "and", so what google is showing you is an authority on "DVD" and a page from that authority with the words "Master" and "Commander" on the page, preferably in the title.
As Edit_G says, exactly what you wanted.