Jonathan Rose’s The Edwardian Temperament 1895-1919 has useful statistics about publication during the period, taken from The Publisher’s Circular, a trade journal.
I’ve taken his figures, and produced a couple of charts which show the effect of the War on fiction publishing. The numbers of titles include reprints as well as new books. I don’t know how much of the decline during the War years was due to a smaller number of new titles, or to a decline in reprints. I think the charts are suggestive about trends, but I’m not sure that they constitute proof of anything much.