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http://www.anglo-american.co.uk currently displays ISBNs in exponential format:
they seem to have fixed the expontial display ... but they have lost a little precision in the ISBN. Those last five digits are not important are they?
There's nothing actually wrong with storing an ISBN in a 64-bit float; there's plenty of room (you don't lose the last 5 digits), and it's more storage-efficient than a string...ISBN-10s can end in X, but there's no reason to store the check-digit. Printing them with standard float-printer is obviously stupid, though...