News that Italian bookseller Rudolf Schönegger has been found guilty of stealing a signed first edition of a Harry Potter book will have sent fans of the boy wizard looking through their copies faster than you can say “he who must not be named”.

Not to read, but because the 55-year old’s conviction is a sharp reminder of the inflated prices paid for first editions of the epic children’s series, especially if JK Rowling has scrawled her name in them. Even signed first editions of her adult debut, The Casual Vacancy, head towards four figures.

The stolen copy of Harry Potter and the Goblet of Fire was tagged at £1,675, a nifty profit for anyone who bought it for its £14.99 cover price 18 years ago, but not as nifty as the profit on one signed copy of her debut, Harry Potter and the Philosopher’s Stone, which fetched 26 times that, thanks to a typo. In 2016, a London-based businessman paid £43,750 for the rare book, which has the word “philosopher” misspelt on the back cover.

It comes as a shock to anyone who cashed in first editions of Rowling’s books 10 years ago amid predictions that Peak Potter had been reached when the last novel in the series, Harry Potter and the Deathly Hallows, appeared. Back then, one critic I know made a tidy £18,000 on a complete set of proofs and first editions. Another pocketed £4,000 for a proof of the Philosopher’s Stone. Kerching! They have watched slack-jawed as prices inflated like a bitcoin bubble. Rowling’s fans’ cult-like devotion is one reason signed first editions remain valuable. But rarity is what makes the real money.

The values of Harry Potter titles have an inverse relation to pagination, thanks to small print runs and typos. The first three books, which, added up, are shorter than the last, always fetch top dollar at auction. A proof of Philosopher’s Stone with the attribution printed as JA rather than JK Rowling sold in 2017 for £10,000. The hardback of book three, The Prisoner of Azkaban, fetches similar prices as the initial print run was stopped midway when it was spotted that “Joanne” rather than “JK” was on the copyright page.

Poor Angus Wilson. Schönegger used a copy of his Late Call as a decoy for his theft. Its value, according to Abebooks, is £6.