Rageboy apparently saw Amazon.com doing an A/B test of citation links between books. It seems to be gone now, but Rageboy's post includes lengthy descriptions of the new feature.
Paul Bausch also noticed the new feature and has a link to a still-active Amazon help page with a little more information about it.
Interesting. And very useful for technical books. I hope they launch it soon.
Google Scholar offers similar functionality for academic papers and books. Seems that Amazon and Google are butting heads more and more frequently lately.
[Rageboy link was via Traffick]
Update: Gary Price points to some interesting work on citation analysis.