The unidentified owner purchased one of the first 500 editions of Harry Potter and the Philosopher's Stone from a vendor at the town's railway station in 1997.
It was expected to attract bids of around Â£35,000 - but has stunned auctioneers at Bonhams by going for Â£56,250.
The rare copy is one of only 500 hardbacks published in the first print run by Bloomsbury, and lists author J K Rowling as Joanne before she had decided to use her initials as a pen name.
There is also a now-notorious mistake in the list of equipment Harry is sent by Hogwarts before his first term - 'one wand' is mentioned twice, an error which is unique to the first editions and was corrected for subsequent print runs.
The purchaser is an American private collector, but a signed copy from the same print run has previously sold for over Â£100,000 at auction.
The Philosopher's Stone was famously rejected by 12 publishing houses before finally being picked up by Bloomsbury in 1997. More than 500million books in the series have been sold worldwide.