This book is the lost season 1907-08 and fits in between books 10 and 11 if you want to read them in that order.
Amelia Peabody and her husband Emerson, along with their son Ramses and foster daughter Nefret, are summoned back to the Lost Oasis, a hidden stronghold in the western desert whose existence they discovered many years ago (in The Last Camel Died At Noon) and have kept secret from the entire world, including their fellow Egyptologists. According to Merasen, the brother of the ruling monarch, their old friend Prince Tarek is in grave danger and needs their help, however it's not until they retrace their steps back to the Oasis, with its strange mixture of Meroitic and Egyptian cultures, that they learn the real reason for their journey. There's no better company on an archaeological expedition than the Father of Curses and the Lady Doctor, their beautiful Anglo-Egyptian ward, and Ramses, the Demon Brother who loves her, as Peters once again demonstrates in the latest historical mystery in this immensely popular series. If you haven't met the indomitable Amelia yet, this intriguing tale is a great place to start! --Jane Adams