Sounds like the sort of job one might get a small software company to do for you...
More seriously, there are services that provide an interface to the ISBN catalog, www.literarymarketplace.com.
On worldcat.com, you can create a URL using the ISBN that will take you straight to a book detail page. That page isn't as very useful because it's still HTML scraping to get the data, but they have a link to download the book data in a couple "standard" formats.
For example, their demo book: http://www.worldcat.org/isbn/9780060817084
Has a "EndNote" format download link http://www.worldcat.org/oclc/123348009?page=endnote&client=worldcat.org-detailed_record, and you can harvest the data from that file very easily. That's linked from their own OCLC number, not the ISBN, but the scrape to convert that isn't hard, and they may yet have a good interface to do it.
