The book follows 13-year-old Parvana as she journeys through a country torn apart by war, befriending homeless children.
"I had a lot of letters from all over the world asking what happened next," said Ms Ellis. "Then the situation in Afghanistan exploded and I went from there."
Published in Britain in November last year, The Breadwinner sold more than 40,000 copies within six weeks. Many schools used it to provide an accessible explanation of the Afghan conflict. Its sequel will be published on September 12.
Royalties from both books will be donated to schools in Afghanistan. Oxford University Press is producing a series of teachers' notes to the novels.