The Idea Factory - The Unbelievable Adventures of Dog Man
A man gets bit by a dog and it gives him superpowers. He goes on several big, unbelievable adventures. But as the book goes on, things start to seem like there might be something wrong. People disbelieving the adventures and straight out contradicting his telling of events. In the end, it's revealed that the main actually contracted rabies and was hallucinating his powers and the adventures he went on.