René Persival’s adventure novel “The Viking’s Grave” feels more like a tightly wound detective story. Something unfortunate happens at Lady Brandon’s house: the famous sapphire “Blue Water” disappears in a mysterious way. Everyone who was in the house comes under suspicion. The visiting nephews of Lady Brandon—Mike, Digby, and John Jest—quietly decide to get out of England, diverting suspicion both from each other and from the rest of the household. Their fates cross again in Africa, in the Foreign Legion, where they will have to pass a brutal test of war and uncover the secret behind the disappearance of “Blue Water.”