One of the best articles I've found describing the use of XML in book publishing:

Integrating A Publishing Business Using XML
by Alex Brown, Griffin Brown Digital Publishing Ltd.

Excerpt:
The scope within the workflow (business process) of an ONIX record gives it a huge potential. A number of major publishers are taking advantage of the fact that ONIX records model the bibliographic details of products right through the workflow, from product approval (or even before) to the product's mature existence.

This enables the ONIX record for a product to act as a pivotal element in an integrated XML workflow. At first the ONIX record is the only data associated with a nascent product, but as production continues more and more data becomes available that can enrich the ONIX record. The real title of the book, to take a simple example, can be drawn from the content XML (and of course titles change during production). Later, in manufacturing, manufacturing data can inform the ONIX record - the true number of pages for example. Finally, post-production, it is the ONIX record which holds the authoritative bibliographic information, metric and pricing information for the now physical products that are being managed in the supply chain.