Beau Geste

A cavalry unit, having crossed the Sahara to relieve a besieged French Foreign Legion fort, arrives to an eerie silence — the enemy has vanished, and the post's walls and ramparts are defended by dead men. The fort's commander, slain by a bayonet through the heart, clutches a letter that links the riddle of the desert massacre to another mystery, the long-ago and far-away theft of a sapphire known as the Blue Water.
It was the scandalous disappearance of the Blue Water that led to the self-exile of Beau, the oldest of the Geste brothers. John and Digby couldn't believe that Beau was a thief and refused to allow him to shoulder the blame alone. Thus all three Gestes turned up in North Africa, among the ranks of the Foreign Legion. Their story of suspense, betrayal, and bravery has inspired several movie versions and remains a favorite with readers who relish a classic adventure. 

Reprint of the John Murray, 1937 edition.