The Scribes from Alexandria: Book 15 by Caroline Lawrence
A desperate quest begins in the Romano-Egyptian port of Alexandria: site of the great lighthouse, the Library and the tomb of Alexander the Great. Codes, riddles, anagrams and hieroglyphics lead the search parties 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?