The memoir by Goncalo Amaral reportedly contains allegations that Madeleine died accidentally in her parents' care, and that they disposed of her body to cover up the death.
The McCanns, both 40, vehemently deny the claims.
Mr Amaral, 48, authorised the decision to name the McCanns as official suspects in their daughter's disappearance. He was widely criticised for ignoring potential leads because of his focus on the couple and his rigid belief that Madeleine died in their holiday apartment in Praia da Luz on May 3 last year.
Many of the lurid smears and bizarre leaks from inside the police inquiry were also rumoured to have come from him. They stopped abruptly when he was removed from the case last October.
Friends of the McCanns said their legal team would study his book, provisionally titled True Lies, and had not ruled out bringing libel proceedings against him.
It is set to hit the shelves three days after Portugal's attorney general is expected to announce the police inquiry will be 'archived'.