King of the Khyber Rifles by Talbot Mundy

Classic adventure novel. According to Wikipedia: "Talbot Mundy (born William Lancaster Gribbon) (April 23, 1879 – August 5, 1940) was an English writer. He also wrote under the pseudonym Walter Galt. Born in London, at age 16 he ran away from home and began an odyssey in India, Africa, and other parts of the Near and Far East. By age 29, he had begun using the name Talbot Mundy, and a year later arrived in the United States, starting his writing career in 1911. His first published work was the short story "Pig-sticking in India", which describes a popular, though now outlawed, sport practiced by British forces. Mundy went on to become a regular contributor to the pulp magazines, especially Adventure and Argosy. Many of his novels, including his first novel Rung Ho!, and his most famous work King of the Khyber Rifles, are set in India under British Occupation in which the loyal British officers encounter ancient Indian mysticism. The novels portray the citizens of Imperial India as enigmatic, romantic and powerful. His British characters have many encounters with the mysterious Thugee Cults. The long buildup to the introduction of his Indian Princess Yasmini and the scenes among the outlaws in the Khinjan Caves clearly influenced fantasy writers Robert E. Howard and Leigh Brackett. Other science-fiction and fantasy writers who cited Mundy as an influence included Robert A. Heinlein, E. Hoffmann Price, Fritz Leiber, Andre Norton, H. Warner Munn, Marion Zimmer Bradley and Daniel Easterman. James Hilton's novel Lost Horizon was partly inspired by Mundy's work."