Truth and justice are very important themes in this book. In fact, one could make the argument that they are the central themes of the book. The titular journey can be seen as an archetypal quest for truth, and the paths that each character takes can be seen as the embodiment of justice.
The scriptures that the main characters are traveling to get literally and figuratively represent truth. The scriptures contain some of the holiest teachings of the Buddhist faith, and adherents believe they are literally true. Figuratively, the represent reaching true happiness and turning away from hedonism and self-absorption.
The theme of justice is most obvious in the premise of the story. All of the main characters have committed one crime or another, and their punishment is to go on an arduous journey to obtain the sutras. Justice is also evident in a positive manner when each character is awarded a post in heaven after successfully completely the quest.