What book about slavery sold more than a million and half copies world wide?

The book about slavery that sold more than 1.5 million copies worldwide is "Roots: The Saga of an American Family" by Alex Haley.

This book, published in 1976, is a fictionalized account of Haley's own family history, tracing his ancestry back to Kunta Kinte, an enslaved African who was brought to America in the 18th century.

The book was a massive bestseller, becoming a cultural phenomenon and sparking renewed interest in the history of slavery. It was later adapted into a miniseries, which also achieved immense popularity.