LOOK inside any book published since 1970 and you will find a number. But perhaps not for much longer. The International Standard Book Number (ISBN), invented in Britain in 1965, took off rapidly as an international system for classifying books, with 150 agencies (one per country, with two for bilingual Canada) now issuing the codes. Set up by retailers to ease their distribution and sales, it increasingly hampers new, small and individual publishers. Yet digital publishing is weakening its monopoly.