His two best known books are "Anthony O'Hara, Knight of Malta" which is an interesting account of a distinguished kinsman of the author, in the writing of which Valentine had access to the Tyrawley papers at Annaghmore House, near Collooney, County Sligo. He also wrote "Russia" in 1925 in collaboration with Nikolai Makeev. This was a much quoted and authoritative account of the late 19th century history of the country.
