Guardian of the horizon - Elizabeth Peters

Unaccountable lost until now, the Emersons' journals chronicling the 1907-1908 season have been discovered, revealing unknown events from Amelia Peabody's remarkable career! Banned from the Valley of the Kings, their excavation at an abrupt end, Amelia and her distinguished husband have returned to England with their 19-year-old son, Ramses, and their foster daughter, Nefret. Ramses is hopelessly and secretly in love with the beautiful young Nefret, and is prepared to flee to Germany to escape temptation.

But a mysterious visitor provokes a change in plans for Ramses and the others. After a decade, Prince Tarek from the Lost Oasis has written to the Emersons, asking for help. His only surviving son is ill with a mysterious malady and his throne may be threatened...Guided only by a crumbling map that took them to the oasis a decade earlier, the Peabody-Emerson clan head for the Sudan. There they will confront forces secretly arrayed against them, and Ramses must come to grips with his feelings for Nefret when she again meets the handsome, chivalrous son.