Perhaps the safest answer is anything they could get their hands on. Most soldiers travelled light to the front and then craved books and magazines once they were embroiled in the stalemate. They would read anything that could take their thoughts off the mud, the rats, the shelling, the smell, the snipers and the prospect of going over the top and charging machine gun emplacements.
The article mentions well-known writers such as John Buchan, Rudyard Kipling and H.G. Wells, and also those who have faded into history, such as Nat Gould (right). It also looks at German and French bestsellers of the time.