Hi gang, Engineering has just deployed a new version of the system, that has a change to ISBNs. In the past, the only check that they made was to make sure that the length of the ISBN was correct. Now they are also doing a checksum validation on all ISBNs. While software engineers know what checksums are, I expect that most people don't. It's basically a method to make sure you haven't entered a bad ISBN, flipped 2 of its digits, etc. If you're curious, I have included the introduction of an article from Wikipedia about checksums at the end of this email. Bottom line is that if you put in an ISBN number and get an error, please double-check the ISBN number that you entered and fix any mistakes. If you're positive that you've entered the correct number and still get an error, please don't enter anything in the ISBN field, click on Continue, and fill out the metadata of the book yourself. Thanks! Carrie Here is the introduction to the Wikipedia article (http://en.wikipedia.org/wiki/Checksum). (By the way, not only do ISBNs have checksums but also credit cards, bank account numbers, etc.) A checksum or hash sum is a fixed-size datum computed from an arbitrary block of digital data for the purpose of detecting accidental errors that may have been introduced during its transmission or storage. The integrity of the data can be checked at any later time by recomputing the checksum and comparing it with the stored one. If the checksums do not match, the data was certainly altered (either intentionally or unintentionally). The procedure that yields the checksum from the data is called a checksum function or checksum algorithm. A good checksum algorithm will yield a different result with high probability when the data is accidentally corrupted; if the checksums match, the data is very likely to be free of accidental errors. Checksum functions are related to hash functions, fingerprints, randomization functions, and cryptographic hash functions. However, each of those concepts has different applications and therefore different design goals. Check digits and parity bits are special cases of checksums, appropriate for small blocks of data (such as Social Security numbers, bank account numbers, computer words, single bytes, etc.). Some error-correcting codes are based on special checksums that not only detect common errors but also allow the original data to be recovered in certain cases.