Harry Buckley, a six-year-old from Otley, spent half his life battling a rare form of cancer.
He passed away in April, but spent the last weeks of his life writing a children's story about his pet hamster, Joey.
"Runaway Joey" has been so successful that it is going for a third print having sold out twice.
The book is being sold to raise money for Nuzzlets farm in North Yorkshire which Harry visited several times after being diagnosed with neuroblastoma in 2011.
Harry wrote the book with his carer Janey, and the family are now considering publishing two other books the pair drafted to raise even more money and awareness.