India: A Sacred Geography. By Diana Eck. Harmony; 576 pages; $27 and £18.99. Buy from Amazon.com, Amazon.co.uk

VARANASI, one of India's holiest cities, has a shrine to Bharat Mata, or Mother India. Built in 1936, its centrepiece is a huge marble map of undivided India, complete with Pakistan and Bangladesh. But it never caught on with pilgrims. That would have pleased John Strachey, a 19th-century British civil servant, who told arrivals on the subcontinent that essentially “there is not and never was an India”. Yet this geographically unwieldy nation-state remains stubbornly united. What binds a country of such immense ethnic, cultural and linguistic diversity? The answer is Hinduism, argues Diana Eck in her sprawling and convincing new book.

The country's landscape is crowded with holy places. There are seven sacred rivers, including the Ganges. On its banks is Varanasi, one of seven holy cities, which itself is guarded by 56 shrines to Ganesha, the popular elephant-headed god. The body of Sati, a goddess, is said to be scattered at 108 sites throughout India. And 12 places across the country claim to have one of Shiva's jyotirlingas—an immeasurable column of the Hindu god's light. The numbers and combinations are endless.

Many of these pilgrimages have been around for centuries. Remarkably, says Ms Eck, the idea of India, called Bharata in Sanskrit, is just as old. “Why should people in this land ever have imagined the land of Bharata—then or now?” she asks. The reason, she writes, is because India “is more than a map, it's a three-dimensional sacred landscape, linked by its storylines”.

Ms Eck has studied Hinduism, travelled in India over many decades and written several books. “India: A Sacred Geography” is a lifetime's work. She makes her argument subtly but persuasively, with numerous, exhaustive and sometimes exhausting examples. In a book that is equal parts travelogue, guidebook and history, she takes the reader across India, telling stories and explaining connections.

Ms Eck starts this spiritual journey by locating India within the cosmos. The ancients imagined it as the southern petal of a lotus island that sits in the centre of the Brahmanda, which translates rather inelegantly to “universe-egg”. Encircling the island are six other islands, like concentric rings, separated by oceans of milk and honey. Each island reputedly has seven rivers, and Ms Eck examines India's for their significance as crossings, both literally and metaphysically.

This book approaches the landscape from the pilgrims' point of view: as mythology rooted in geography. Ms Eck travels to the sites of Shiva's many manifestations, picks up the pieces of the dismembered goddess, and visits the temples dedicated to Vishnu. In Braj, in north India, she relates immersive reconstructions of stories from Krishna's life which pilgrims re-enact. Finally, she traces the journey made by Ram as he travelled from Ayodhya in the north to Lanka (presumed to be present-day Sri Lanka) in the south, navigating the length of the country.

It is only in the last chapter that Ms Eck remembers that India is not just a religious notion but a political reality as well. Sacred geography has been responsible for one of modern India's worst atrocities. In 1992 militant Hindu activists demolished a 16th-century mosque in Ayodhya, claiming that it was the birthplace of the man-god Ram. Sponsored by the Bharatiya Janata Party (BJP), which successfully used it as a political platform, this act sparked some of India's most brutal riots since partition. The BJP has since toned down its Hindu chauvinism, but it still sends its leaders on Hindu-themed campaigning tours.

Ms Eck's book is not an easy read, but it is a rewarding one. She makes a compelling case for India as an integrated, if fractious, land of culturally heterogeneous Hindus. Indeed, this spiritual relationship with the land has seeped into the other national religions—Muslims, Sikhs, Christians and others have their own pilgrimage sites dotted throughout the landscape—which both enriches and complicates the soil on which today's state is built.