The Primal Blueprint: Reprogram your genes for effortless weight loss, vibrant health, and boundless energy. by Mark Sisson.
This book does an excellent job of explaining how the modern western lifestyle and diet is leading to chronic illness. It then offers solutions for the common pitfalls of diet and exercise. Technically this is a Paleo Lifestyle book, but I appreciate how well it explains everything instead of just asking you to blindly trust its advice.
Please give it a read and you will likely never want to return to your unhealthy lifestyle.