Summon the Bright Water — Geoffrey Household

On a walk in the woods, a historian is drawn into a conspiracy of murder

Piers Colet is about to fly to Spain when his plane's engine catches fire. His research trip delayed, he decides to spend the week exploring the banks of the Severn, the ancient river that winds through the backwoods of Wales. Searching for a place to bed down for the evening, he finds himself in Simeon Marrin's commune, where a group of people disgusted with the course of modern civilization have gathered to wait for the coming end of the world. Here he meets the charming Elsa, who intrigues him only slightly less than the rumors that the Severn runs rich with gold.
 
When a trip into the water with Marrin nearly costs Colet his life, the economic historian goes to ground, taking shelter in the mysterious Forest of Dean. Where ancient Romans once fought, where Druids once worshipped, Colet must now learn to kill.