To check the sales rank of Oracle books on Amazon (that would be the e-commerce site that sells books, not the river), you can use this link, or follow this breadcrumb from Amazon.com - Books > Subjects > Computers & Internet > Databases > Specific Databases > Oracle > General.
The above link opens the page with the books sorted by their "bestselling" status.
This link was originally posted on Tom Kyte's blog (link to post, link to blog), where his book, "Expert Oracle Database Architecture: 9i and 10g Programming Techniques and Solutions", is at #2 - "eBay Powerseller Secrets:Insider Tips from eBay's Most Successful Sellers" is at the number 1 position, why I don't know.
Michael Armstrong-Smith's "Oracle Discoverer 10g Handbook" is at #14 (link to book on Amazon.com, link to Michael's blog).
BTW, Michael's ACE (link to page on Oracle.com) profile is now up on Oracle.com here.
Finally, if you want to go to the Oracle Store on Amazon.com it seems you can directly type http://www.amazon.com/oracle/ (which redirects you to this URL).