In the above case the book number is an incrementing number (which I probably didn't make clear) that is used by people to reference the book, not the system. Internally books are given UUIDs on creation, so the system can reference them, but UUIDs are not human friendly, so we also generate the book number.
UUIDs really are a life saver in these kind of systems, I honestly can't live without them anymore.
Ahhh, thanks I must have missed that on my first read!
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