* A History of Wales (with Wynford Vaughan-Thomas, 1968)
* Dylan Thomas: His Life and Work (1964)
* World War I: A People's History (2005)
* The Battle of the Somme: A New History (2006)
* 1918: Voices of the Great War (2018)
Rees has also written several novels, including:
* The Last Englishman (1973)
* The Devil's Piper (1977)
* The White Bird of Rhiannon (1980)
* The Welsh Tapestry (1984)
* The Land of the Living (1988)
In addition to his work as an author, Rees has also been a journalist for a number of publications, including The Sunday Times, The Daily Telegraph, and The Times Literary Supplement. He has also worked as a broadcaster for the BBC and ITV.
Rees is a Fellow of the Royal Society of Literature and a Member of the Gorsedd of Bards of Wales. He has been awarded the OBE for his services to literature.