All three books are pretty interesting, although I did skip some of the biographical material to get to the physiology and running bits. It's not that Heinrich's life wasn't interesting, it's that he doesn't really explain the most interesting part. He was a little boy in a war zone; it's easy to understand how that would affect one's outlook, and Heinrich explains consequences one might not think of. On the other hand, his successful parents abandoned him for most of his childhood at an orphanage, so they could pursue their careers. This I found frustrating. Most parents could not do this. Why did his parents reach this decision? How did Heinrich feel about this? Considering that the orphanage wasn't a particularly pleasant place, what sort of relationship did he have with his parents as an adult? On this, the author is silent.
The best bits are the running bits, which are well-written and fun, and the physiology bits, which are somewhat scattershot but thought-provoking. The book meanders. I imagine it like sitting by the fire with Heinrich on a cold Vermont evening after a few glasses of wine. It's interesting, but I can't help checking my watch.
So why do we run? Heinrich argues that we're built to run long distances so that we can hunt by running prey to exhaustion. Despite his interest in animal models including migratory birds, Heinrich doesn't consider that being able to cover great distances would be equally useful in foraging and migrating.
Regardless, the idea that humans are built to continuously run distances beyond that of almost all land animals was controversial when this book was published, and less so now. Why We Run is still relevant despite its problems, because its key message is an important idea.