Two of the 100+ novels by early spy novelist E. Phillips Oppenheim will be coming out this August. The Great Impersonation is a WWI doppelganger story with a disgraced English aristocrat and a German baron swapping places. The Spy Paramount from the 1930s takes place in Fascist Italy and features a hunt for a secret weapon. Lovely cover.
Finally, Mystery in White by J. Jefferson Farjeon is a classic Christmas mystery from the 1930s, with a group of snow-bound train passengers trapped together in an empty house over the season of goodwill. Available from November.