From Goodreads:

In this second Ulysses Donaghue mystery, PI Ulysses F Donaghue finds himself investigating a suspicious death while on a walking tour in the Vallée des Prêtres in the Franco-Italian Alps, site of Bronze Age rock drawings discovered 20 years earlier by celebrated archaeologist, Sir Hilary Compton.
When new drawings and an unexpected hieroglyphic are discovered by young archaeologist, Oliver Hardcastle, a motley group of academics, journalists and tourists assemble at the site to witness Sir Hilary's authentication of the new discovery.
But when Hardcastle's body is found floating in the treacherous Lake of Shadows, Donaghue abandons Bronze Age deliberations in order to seek out the murderer.
An atmospheric classic Christie style whodunit with an unexpected twist in the tail.