This is the LibraryThing founder. We have nothing to offer here, so I hope my comments will not seem self-serving.
First, the comment about Amazon, ASINs and ISBN numbers is wrong in a number of ways. In almost every circumstance where a book has an ISBN, the ASIN and the ISBN are the same. ISBNs are not now 13 digits. Rather, ISBNs can be either 10 or 13. Ten-digit ISBNs can be expressed as 13-digit ones starting with 978, which means every ISBN currently in existence has both a 10- and a 13-digit form. There are all sorts of libraries available for converting between ISBN10 and ISBN13. Basically, you add 978 to the front and recalculate the checksum digit at the end.
ISBN13 was invented because publishers were running out of ISBNs. In the near future, when 979-based ISBN13s start being used, they will not have an ISBN10 equivalent. To my knowledge, there are no published books with 979-based ISBNs, but they are coming soon. Anyway, the long and short of it is that Amazon uses the ISBN10 form for all 978 ISBN10s. In any case, whether or not Amazon uses ten or thirteen-digit ASINs, you can search Amazon by either just fine.
Personally, I wouldn't put ISBN DB at the top of your list. ISBN DB mines from a number of sources, but it's not as comprehensive as Amazon or Google. Rather, I'd look into Amazon—including the various international Amazons—and then the new Google Book Data API and, after that, the OpenLibrary API. For non-English books, there are other options, like Ozone for Russian books.
If you care about the highest-quality data, or if you have any books published before about 1970, you will want to look into data from libraries, available by Z39.50 protocol and usually in MARC format, or, with a few libraries in Dublin Core, using the SRU/SRW protocol. MARC format is, to a modern programmer, pretty strange stuff. But, once you get it, it's also better data and includes useful fields like the LCCN, DDC, LCC, and LCSH.
LibraryThing runs off a homemade Python library that queries some 680 libraries and converts the many flavors of MARC into Amazon-compatible XML, with extras. We are currently reluctant to release the code, but maybe releasing a service soon.
