In Jainism, the best single book to understand life and its purpose is the Tattvartha Sutra (also called Tattvarthadhigama-Sutra) by Umāsvāti. It is a concise, systematic summary of Jain philosophy—discussing the soul, karma, ethics, conduct, and the path to liberation. It is respected across both Digambara and Śvetāmbara traditions, though commentaries and emphasis can differ between traditions.

If you’re new and want nearby alternatives that also illuminate life from a Jain viewpoint, you might explore:

• Kundakunda’s texts like Niyamasāra and Samayasara for a devotional and practical view of the soul and conduct.
• Acharanga Sutra for a broad ethical and doctrinal foundation.

Note on differences: Tattvartha Sutra is accepted by both traditions, but its commentaries and the way some terms are interpreted can vary between Digambara and Śvetāmbara schools.