Christopher Tolkien has spent 30 years working on The Children of Hurin, which The Lord of the Rings author started in 1918 and later abandoned.
Extracts from The Children of Hurin have been published before.
JRR Tolkien's The Lord of the Rings trilogy has sold more than 50 million copies and was adapted into three hugely successful films.
"It has seemed to me for a long time that there was a good case for presenting my father's long version of the legend of The Children of Hurin as an independent work, between its own covers," Christopher Tolkien said in a statement.
The story involves the elves and dwarves that feature in much of Tolkien's work.
He died in September 1973, aged 81.
