Here are a few contenders:
1. The Bible: This is undoubtedly one of the most widely distributed and translated books in history. It has been on various bestseller lists for centuries, but it's difficult to quantify its exact duration.
2. "The Guinness Book of World Records": This book has consistently appeared on bestseller lists for over 60 years, making it a strong contender for the longest-running bestseller.
3. "The Little Prince" by Antoine de Saint-Exupéry: This classic children's story has been a bestseller for decades and continues to enjoy enduring popularity.
4. "The Lord of the Rings" trilogy by J.R.R. Tolkien: This epic fantasy series has remained a popular choice for readers for over 70 years and consistently appears on bestseller lists.
Ultimately, it's challenging to pinpoint one definitive answer. Different bestseller lists vary in their criteria and historical record-keeping, making it difficult to accurately compare durations.
However, the books mentioned above are strong contenders for the title of longest-running bestseller, each with a long and illustrious history of sales and popularity.