xISBN (Web service) should be of interest to topic mappers.
From the website:
The xISBN Web service supplies ISBNs and other information associated with an individual intellectual work that is represented in WorldCat. Submit an ISBN to this service, and it returns a list of related ISBNs and selected metadata.
ISBNs are related to each other in WorldCat using an algorithm developed by OCLC Research. The algorithm restructures WorldCat bibliographic records to conform to the FRBR conceptual model for information objects. For instance, rather than requiring an end user to traverse multiple records that represent many different manifestations of a book—including printings, hardback or paperback editions or even filmed versions—”FRBRized” WorldCat information allows that user to review a core record that lists all manifestations.
The xISBN Web service queries database tables in WorldCat created by the FRBR algorithm.
I got there from e-Book Finder, a nifty site build on top of xISBN, that tries to find electronic versions of books. Not necessarily free electronic versions, just electronic versions.