A spiritual guide to self-discovery and fulfillment by Hermann Hesse. It's very short and goes straight to the point. Hesse isn't blindly preaching frugality and asceticism, but instead shows the process behind achieving such an end.
I would say Siddhartha is a shorter, more condensed version of Narcissus and Goldmund. The plot structures and themes explored in both novels are quite similar. The main takeaway, though, is that wisdom cannot be taught — something that translates quite well when trying to teach kids about the world.