3 ideas
Two copies can carry identical text and still be different books. Their authenticity and value sit in material particulars: paper stock, binding, printer's marks, colophons and small variations between the engraved plates. The bibliographic detective therefore works by side-by-side physical collation. He assumes a forgery or deliberate variant will betray itself in a detail no one thought to copy, not in what the book says.
A well-read investigator who spots a few literary echoes starts forcing every event into the familiar story. Corso casts the people he meets as Milady, Rochefort and the Musketeers, and so wrongly concludes that the Dumas manuscript and the devil book belong to one conspiracy. In fact they are unrelated. The more patterns a reader knows, the more readily he invents connections, and the reader's own imagination becomes the main source of error.
For the obsessive collector, a book's worth comes from scarcity, provenance and completeness rather than from what it says. That shift makes possession an end in itself, one that can justify destroying other copies to make one's own unique. Seen this way, the rare-book trade is driven by the same logic as relic worship and the market in fetish objects.