This has been one of my favorite books for over a decade now! I first read it in early high school--I think back then I saw it most as an incredibly inspiring story with characters that I wanted to be and know in real life. I read it again in college and saw it as a story about someone who gives their entire existence to focus on a goal, that happened to be running in Cassidy's case. I love how well it captures the mindset of a distance runner, as well as the mindset of anyone obsessively chasing an extreme goal. I've seen a lot of criticisms of Once A Runner, saying it's not relatable to most runners who see running as not only a sport of being faster than everyone else. I wholeheartedly disagree with those criticisms because I think they've missed the point of the book. This book isn't about anyone who runs, but about someone who defines themselves as a runner. Obviously, running isn't only about racing. But everyone can relate, at some point in their lives, to the desire (or at least curiosity) of honing a craft to perfection. And Once a Runner tells that story. 
This most recent time re-reading I finally use a reading log app, so I have the opportunity to share a review. At 25 now, I suppose I don't call it my favorite book anymore. But I still think about the Trial of Miles at least once a week, even though I don't run many miles these days. I feel the nostalgia of the book. This is one that I will re-read throughout my life and learn different things about myself each time.