Of the five books within reach only two share a format for the ISBN, they all start "978-n-" and end with dash check digit but in between is 2-6, 3-5, 6-2 and 5-3. A quick trip to the "I need to build two new bookcases" pile shows several 4-4 although no 1-7 or 7-1. It's not even consistent by publisher, the two nearby 3-5s are from Baen, but so is (at least) one of the 4-4.
First...
Of the five books within reach only two share a format for the ISBN, they all start "978-n-" and end with dash check digit but in between is 2-6, 3-5, 6-2 and 5-3. A quick trip to the "I need to build two new bookcases" pile shows several 4-4 although no 1-7 or 7-1. It's not even consistent by publisher, the two nearby 3-5s are from Baen, but so is (at least) one of the 4-4.