First, the name 'Siddhartha' is misleading because it's also the name of young Gotama who became enlightened later on to be called Buddha, the awakened one. You thought you're reading about the actual Siddhartha, the Buddha-to-be or closely related one but it's completely author's wild imagination about a young Hindu boy 'Siddhartha'. I understand it's a novel so you can't just take its face value.
Overall, there is a lot of injection from Hinduism and although you would think this would inform you of some Buddhist ideology, I think it's missing many of the fundamental essence of Buddha teachings.
When you watch a documentary movie, would you expect to see fictional characters and make-up conversations? Of course, not. And this book apparently claimed to be novel so you can't be upset when author made up his own twists and turns. But why did he have to use the actual name and closely depicting the real life of Siddhartha yet with many misleading ideas? One would get confused and thought it as real.
'Samsara and Nirvana are only words' said Siddhartha. Author created Siddhartha starting with a powerful resourceful intelligent Brahmin's son, a young Hindu boy to become calm Ascetic, wood dwelling Samana, then turned Lover Luster, Rich Merchant. Later as Ferryman, then became Father. Going through life with deep thoughts and listening to river eventually made Him EQUAL to Gotama, without teachers, know all things by Himself.
As a learned Buddhist, you know those are not true. Should I even say, it's insulting?
If actual Siddhartha or Gotama Buddha didn't exist, I don't care people playing with their mind and writing whatever they want. His life experiences, thought processes, reflections, revelations, or contemplations are good and valid but since there once was a real prince named Siddhartha who became Gotama Buddha, I hope people after reading this book should continue to be curious about the real Siddhartha and his actual life events, don't stop at fictional one like this.