This novel is about our hero Siddartha's search for spiritual truth and his eventual discovery of a personal philosophy, which leads him to enlightenment and a life free of fear and desire.
Siddartha is also an allegory that can apply to all of us. It teaches us how to have a liberating philosophy for life, which most westerners will very different from their usual modern, materialistic perspectives.
Should you decide to read this wonderful work, it is worth noting from the outset that, though it contains religion, it does not preach the tenets of any faith. It is purely and simply about philosophy.
This book is worth reading for it's own merits, but if you read Hesse's Demian and Steppenwolf first you will get even more benefit from it. These three novels changed my life!