"Building Microservices, 2E" never matched "Building Microservices". Key the
strip on the trailing "e" so 2E, 5e and 3 Ed are caught, while a bare number
leaves "Catch 22" and "Blade Runner 2049" alone. Stored keys are recomputed.
DC:identifier was validated verbatim, so hyphenated and urn:isbn: forms never
reached the checksum and every non-ISBN identifier was discarded. Normalise
first, and name whatever survives.
Reduce a title, an author and an identifier to a single comparison key, so
two copies of one book can be recognised by equality rather than by a
similarity score.