Think you have to ask yourself what XML really is.
IMO XML is ultimately a techology for exchanging data between systems in a platform independent manner.
For an e-commerce site, I'd investigage "web services" with either SOAP or using an approach like Amazons REST based XML service. The idea would be to make your sites content available for other sites to display, there by enabling sales to begin on other websites and effectively increasing your market.
On the books front, Wrox's "Pro PHP XML" (which I have and like) and New Riders "PHP and XML" both have good reputations.
Generally I'd say though for what you want to do, consider making your content available as both (X)HTML and XML (via a slight modified URL). Displaying the latter should be basically the same as the HTML but without any "user interface" (navigation etc.). Once you've got that far, investigate WSDL which basically should provide you a means to "index" your XML.