A magic ISBN markup similar to the one on Wikipedia would benefit several of the StackExchange network sites created over the last year, including cstheory and math. It would also be useful for 1.0 sites like mathoverflow, physics, and physicsoverflow. People need to refer to textbooks and monographs all the time on these sites.
isbn: <number> into
http://en.wikipedia.org/wiki/Special:BookSources/<number> which allows someone using the site to find the book in lots of different places (including in library catalogues, various online retailers, and online). It is also possible to set up the magic tag to go to one specific place always, bypassing the BookSources page. Here is what it looks like: http://en.wikipedia.org/wiki/Special:BookSources/978-0-89871-432-6
Right now on SE sites there is a completely ad hoc way to link to books, using Google Books (which seems to give different results for different people), country-specific Amazon sites, results from Google or Google Scholar, or third party sites like isbn.nu.
Note that affiliate-tagged links to various online retailers are typically not used on research-oriented sites, as the book reference is typically made by someone who does not stand to benefit from the book being bought. The aim is to do research, not to sell books. A generic linking mechanism is therefore appropriate, especially if other communities (historians, biologists, philosophers, ...) embrace the StackExchange model of Q&A sites.
As has been pointed out in add an <isbn:> tag? a magic ISBN tag would allow the user to configure "preferred retailer or library" and would increase site stickiness.
I am aware of the long history of autoconverting Amazon affiliate tags. This proposal is orthogonal. If the user agrees, SE affiliate links could also be added to retailer links that are generated by the magic tag.
See related discussions: