Sam Leith's guest on this week’s Book Club is the bioarchaeologist Cat Jarman, whose fascinating new book River Kings spins a global history of the Vikings out of a single carnelian bead found in a grave in Repton. Cat tells him how much more there was to the Viking culture than our traditional image of arson, rape and pillage in Northumbria - showing how 21st century techniques have helped to expose a culture that raided and traded from Scandinavia as far as Baghdad and Constantinople, and may even have been the ancestral population of the Russian heartland. Plus: real-life Valkyries, slavery and human sacrifice. You never learned all this from How To Train Your Dragon...