The first time I read Siddhartha, I knew only the very basics of Buddhism and Hinduism. Since Siddhartha is a common Indian name (or at least I've heard of it) and all spiritual concepts in the book seemed like Hinduism, I did not associate it as a "Buddhist" book in the slightest, especially since he meets and rejects the Buddha in the course of the novel. I still fail to see how people confuse it as a biography of the Buddha. You would have to turn off your mind to get that impression.
No, for me, Siddhartha the novel is simply a novel that is inspiring - a translation of the "Book of the Heart" that transcends dharma, history, and all the sankara's we generally attach to any combination of ink and paper (or pixels now-a-days). It did however, give my abstract spiritual feelings some form that I could work with. I looked into Hinduism, but quickly found my true home in Buddhism.
There are levels of dhamma - the formal teachings of the Buddha, which are not at all part of Herman Hesse's novel - and the real heart of the wisdom-compassion, which Hesse does a fine job in bringing to life IMO.
Does anybody find it ironic that Siddhartha's one and only criticism of the Buddha's doctrine is that because enlightenment can not be taught the description is inadequate, yet Siddhartha goes on to have many informal teachers (including a river and a whore) and a semi-formal teacher in the ferryman that he lives with by the river? I don't believe this is an irony that Hesse left in by neglegence. It shows that we do need teachers, even when they are not formal. We all learn from experience and from people. A few rare individuals can do it without a formal teacher (Siddhartha, the Buddha, the ferryman), but the rest of us need formal methods to cut off our coarser defilements. Even then, we must realize that a good teaching only narrows the search, and genuine Truth lies outside the domain of a formal teaching.
And let us not forget that Siddhartha's training was not complete until he learned how to love. Only then could he see the people he ferried across the river, not as children, but as humans full of dignity and suffering. Though he was respectful and free of ill-will throughout the novel, it is only after losing his son that he develops compassion. That was his final humility, losing the pride of dignity and conceit, and he never would have gotten there without the ferryman (a good companion).
Anyway, that's my take. I'm not expecting to change any minds about the novel. And yes, I read it again just a couple months ago and I was just as moved as the first five times I read it. It is not a book about Buddhism. But it is a book about sila, samadhi, and panna.
Sotthī hontu nirantaraṃ - May you forever be well.