Rees's most acclaimed novels include The Shrieking Pit (1919), a suspenseful murder mystery set in Norfolk, and The Moon Rock (1922), a tale revolving around a stolen sapphire on an island off the Cornish coast. Other significant works include The Hand in the Dark (1920), The Hampstead Mystery (1916) and The Tragedy of the Silent Pool (1919), the latter two co-written with John R. Watson, as well as The Dartmoor Enigma (1933).
In 1927, Rees co-founded The Mystery, a short-lived magazine devoted to detective fiction. Although less famous today than contemporaries such as Agatha Christie, Rees earned praise for his clever puzzles, vivid settings, and engaging narratives. His novels have been reprinted by specialist publishers, and critics credit him with a distinctive place in the development of British crime fiction.