You always find such interesting things on your walks. 'The Spruce King' definitely sounds like a ripping yarn - although the title makes me think of the troll king in Hans Christen Anderson rather than derring do. I do love the idea of random books released into the wild like that, and I'm glad you rescued it from the road.
Wonder how it got there? Flung from a car by someone maddened by the purple prose or early 20th century attitudes to forestry???