"The Unlikely Pilgrimage of Harold Fry" by Rachel Joyce, "The Power of Now" by Eckhart Tolle, "Why Can't I Meditate" by Nigel Wellings and "The Happiness Project" by Gretchen Rubin are a few non-religious inspirational books. Each of these books takes a different approach to inspire the reader.
"The Unlikely Pilgrimage of Harold Fry" focuses on relationships, resilience and love based on the lessons learned by a man who walked across England. "The Power of Now" focuses on how a person can overcome depression, illness and everyday problems through the power of mindfulness. "The Happiness Project" tells the story of a woman who used science and ancient wisdom to actively live a happier life for a single year. Each book has aspects of spirituality but does not follow a specific religion.