Complex sentence: In 1810, she published her first book: a biography of her friend Fernow, who had died two years before, which she wrote with the intention to pay his heirs' debts with his editor.
Split into simpler sentences: In 1810, she published her first book: a biography of her friend Fernow, who had died two years before. She wrote it with the intention to pay his heirs' debts with his editor.