Note: I'm the LibraryThing guy, so this is partial self-promotion.
Take a look at this StackOverflow answer, which covers some good ways to get data for a given ISBN.
To your issues, Amazon includes a simple DDC (Dewey); Google does not. The WorldCat API does, but you need to be an OCLC library to use it.
The ISBN/UPC issue is complex. Prefer the ISBN, if you can find them. Mass market paperbacks sometimes sport UPCs on the outside and an ISBN on inside.
LibraryThing members have developed a few pages on the issue and on efforts to map the two:
- http://www.librarything.com/wiki/index.php/UPC
- http://www.librarything.com/wiki/index.php/CueCat:_ISBNs_and_Barcodes
If you buy from Borders your book's barcodes will all be stickered over with their own internal barcodes (called a "BINC"). Most annoyingly whatever glue they use gets harder and harder to remove cleanly over time. I know of no API that converts them. LibraryThing does it by screenscraping.
For an API, I'd go with Amazon. LibraryThing is a good non-API option, resolving BINCs and adding DDC and LCC for books that don't have them by looking at other editions of the "work."
What's missing is the label part. Someone needs to create a good PDF template for that.
