Simon Smith, 57, unearthed the rare haul of letters and drawings exchanged between the creators of the famous bear tucked away inside carrier bags in Malvern.
His father Leslie, who died aged 96 in 2023, worked in the publishing industry all his life – but his family knew nothing about the hidden archive in his attic.
The collection contains first-draft manuscripts by the author AA Milne, including his poem Wind on the Hill.
It also includes sketches, autographs, drafts and corrected proofs for Now We Are Six and The House at Pooh Corner.
An original pen and ink drawing of Pooh and Piglet walking side by side in the snow is also featured along with original mock-ups for Christopher Robins Birthday book.
Some of the documents have not been seen since 1926 and were unearthed as Leslie’s children went through his estate.
There was also a signed postcard from Lord of the Rings author JRR Tolkien.
Mr Smith said: “We just didn’t know they existed so we were pretty gobsmacked.
“We had been clearing out the attic and found a plastic carrier bag full of letters.
“The Tolkien postcards must have been there since 1957 as my dad’s firm handled the launch of Lord of the Rings.
“I wasn’t too shocked to find those as I knew about his association with Tolkien but the Winnie the Pooh stuff was really surprising.
“I have three siblings and none of us knew they were there.”
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The items went up for auction in Stourport on Thursday (January 16).
Auctioneer and BBC Antiques Roadshow star Will Farmer said they uncovered “previously unseen private conversations and ideas about some of the world’s most famous children’s publications.”
He added: “Within everything that everybody all around the world knows about Milne, EH Shepherd and Winnie the Pooh, these are to my mind probably one of the last little pieces of the puzzle that will complete the whole story.”
Clive Farahar, a book specialist who helped appraise the collection, said: “This is an incredible discovery in the world of Winnie the Pooh, especially when it was thought all documents were accounted for.
“It has been an absolute pleasure to read through these rare, unique and personal letters and documents which give further insight into our favourite honey loving bear.”