The 15th century treasure was found gathering dust in the museum cellar among a slew of other items.
“There is only one other known copy in the world, at the Royal Library in Copenhagen,” said Ulf Johansson, head of Sundsvall libraries, to the local Dagbladet daily.
The Sachsenspiegel (literally: Saxon mirror) is a law book in Low German, written by hand in the early 1200s of the Saxon nobleman Eike von Repgau.
It is described as the most important German book of law from the Middle Ages and was in force in force in parts of Germany until the 1900s.
The rare itme is reported to be in good condition and is now housed at the National Library (Kungliga Biblioteket – KB) in Stockholm.
The book formed part of a donation in the early 1900s by the heirs of a doctor to the medical school in Sundsvall.