Answer
Buddhism (founded by the Buddha), Islam (founded by Muhammad) and Christianity (founded by Jesus Christ) are religions established by single individuals. Because of that, each of them has essentially one — or just a few — sacred books. Hindu Dharma does not have a single, exclusive sacred book.
As explained earlier, Hindu Dharma is a kind of science — adhyatma-vijnana, the science of the inner life — and it is a vast collection of principles and practices discovered by many rishis, avataras, saints, and thinkers.
Science itself has no single book. Physics has its own books, chemistry has its own books, biology has its own books. In the same way, Hindu Dharma has separate sacred texts for dhyana-yoga (meditation), for yajna and Vedic rituals, for tantra-sadhana, and for bhakti — different paths, different scriptures.
Why so many? Because Hindu rishis developed many paths (panthas) to one single goal — moksha. Different paths require different books. And because Hindu Dharma is the world’s most ancient religious tradition, its rishis, saints, and thinkers were given an extraordinarily long span of time to explore inwardly in many directions. That is why so many traditions of knowledge, philosophy, sadhana, and worship developed within it.
Yet among all these paths and scriptures, the Bhagavad Gita stands out as the most universally honoured and widely accepted text of adhyatma-jnana (inner knowledge). All Hindu traditions accept the Gita as their shared, primary sacred book. According to Hindu belief, it is the very word of God. In a brief and simple form, it presents the knowledge of how to make this life noble and how to attain moksha. Dhyana, bhakti, jnana, karma — all the principal paths to moksha — are gathered into this one scripture.
The Vedas, too, are foundational scriptures of Hindu Dharma and are considered apaurusheya (not of human authorship); they are therefore also shared, primary scriptures of Hindus. The Vedas, however, deal more with hymns to the devas, yajnas, and ritual practice. For the knowledge of adhyatma specifically, the Bhagavad Gita is the most beloved and most widely accepted.
If one scripture stands above the rest, then naturally there must also be ideal individuals who embody Hindu Dharma in life and in teaching — which brings us to the next question, about the two greatest figures of Hindu Dharma.