I guess it depends on the circumstances. If you have the field available already, you could do it with pure JavaScript. Alternatively, you could hook up a Web Service and execute it with jQuery to get the related data. Personally, once you go down this route, I would drop the DetailsView and hand-code the form instead.
Can you please post questions not directly related to the book in a more general ASP.NET category:
http://p2p.wrox.com/asp-net-4-539/ ? That way, you'll attract more viewers, while this category stays easy to use for readers of the book.
Cheers,
Imar