The Roman Mysteries: The Scribes from Alexandria: Book 15 by Caroline Lawrence
A desperate quest begins in the port of Alexandria: site of the great lighthouse, the famous Library, and the tomb of Alexander the Great.
Codes, riddles, anagrams and hieroglyphics lead the young detectives down the river Nile to pyramids and sphinxes, temples and tombs, crocodiles and hippos. But what lies at the end of the journey? Treasure? Or death?