Siddhartha is a good one.
My personal favorite would be The River Why by David James Duncan. It's such a great story, starts out as somewhat of a coming of age novel, but as the story progresses, it becomes very philosophical, focusing on the protagonist's search for meaning in life, and introducing a plethora of fantastic characters. It's actually my favorite book of all time, probably a bit biased because my Mom read parts of it to me before she died, which led me to reading it on my own. Nevertheless, it's an extremely well-written book.