March 28, 2016 10:30 AM EDT

In 1977, a Dutch family revisited a book their parents had bought them shortly after World War II and discovered something remarkable inside. Their copy of Grimm’s Fairy Tales had two signatures on the title page: those of Anne Frank and her sister, Margot Frank.

The book had been left behind by the Frank family when they fled their Merwedeplein apartment to hide in the now-infamous annex where they tried to escape persecution. Sometime after that—after the Frank family had been found, after Anne and Margot died at Bergen-Belsen—the book ended up in an Amsterdam bookstore, and subsequently in this Dutch family’s home.

Read more: TIME’s Original Book Review for Anne Frank’s Diary

When the family members (who wish to remain anonymous) discovered that the book had belonged to the famous diarist, they wrote to Anne’s father, Otto, who had shepherded his daughter’s diary to publication after her death. He urged them to keep the book, explaining “it would please me if you keep the book for your daughter in memory of my children.”

Here is the full text of Otto Frank’s letter, in a translation provided by Swann Auction Galleries:

Now, nearly 40 years after the book was rediscovered, it and the letter will go up for auction at Swann on May 5.

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Contact us at letters@time.com.

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