Birdwatcher's quarry

AKA The Three Beans From the book jacket: "Alan Power, British intelligence agent and friend of Tommy Hambledon, had come down to the little French village of Arnage-sur-Lire on the trail of something very big. Tommy had given Power two assistants, agents Campbell and Forgan, and his blessing; the next thing Tommy heard of Power was the news of his death in a motor accident ... an "accident" to which Campbell and Forgan had been witnesses. It didn't take long to determine that Power had been shot in the head while driving his racing car around a dangerous curve. And it wasn't long before still another witness declared himself: and elderly ornithologist who rapidly became a figure of mystery." Tommy sets out to find his friend's killer.