Interesting reading though not a rare book....
The Rise of Islam and the Bengal Frontier, 1204–1760
By Richard M. Eaton
Published by University Of California Press - 1993

Book Extract :
Sometime in 1243–44, residents of Lakhnauti, a city in northwestern Bengal, told a visiting historian of the dramatic events that had taken place there forty years earlier. At that time, the visitor was informed, a band of several hundred Turkish cavalry had ridden swiftly down the Gangetic Plain in the direction of the Bengal delta. Led by a daring officer named Muhammad Bakhtiyar, the men overran venerable Buddhist monasteries in neighboring Bihar before turning their attention to the northwestern portion of the delta, then ruled by a mild and generous Hindu monarch. Disguising themselves as horse dealers, Bakhtiyar and his men slipped into the royal city of Nudiya. Once inside, they rode straight to the king’s palace, where they confronted the guards with brandished weapons. Utterly overwhelmed, for he had just sat down to dine, the Hindu monarch hastily departed through a back door and fled with many of his retainers to the forested hinterland of eastern Bengal, abandoning his kingdom altogether.

This coup d’état inaugurated an era, lasting over five centuries, during which most of Bengal was dominated by rulers professing the Islamic faith. In itself this was not exceptional, since from about this time until the eighteenth century, Muslim sovereigns ruled over most of the Indian subcontinent. What was exceptional, however, was that among India’s interior provinces only in Bengal—a region approximately the size of England and Scotland combined—did a majority of the indigenous population adopt the religion of the ruling class, Islam. This outcome proved to be as fateful as it is striking, for in 1947 British India was divided into two independent states, India and Pakistan, on the basis of the distribution of Muslims. In Bengal, those areas with a Muslim majority would form the eastern wing of Pakistan—since 1971, Bangladesh—whereas those parts of the province with a Muslim minority became the state of West Bengal within the Republic of India. In 1984 about 93 million of the 152 million Bengalis in Bangladesh and West Bengal were Muslims, and of the estimated 96.5 million people inhabiting Bangladesh, 81 million, or 83 percent, were Muslims; in fact, Bengalis today comprise the second largest Muslim ethnic population in the world, after the Arabs.

How can one explain this development? More particularly, why did such a large Muslim population emerge in Bengal—so distant from the Middle East, from which Islam historically expanded—and not in other regions of India? And within Bengal, why did Islamization occur at so much greater a rate in the east than in the west? Who converted and why? At what time? What, if anything, did “conversion” mean to contemporary Bengalis? And finally, between the thirteenth and eighteenth centuries, in what ways did different generations and different social classes of Muslims in Bengal understand, construe, or even construct, Islamic civilization? In seeking answers to these questions, this study explores processes embedded in the delta’s premodern history that may cast light on the evolution of Bengal’s extraordinary cultural geography.

Bengal’s historical experience was extraordinary not only in its widespread reception of Islam but also in its frontier character. In part, the thirteenth-century Turkish drive eastward—both to Bengal and within Bengal—was the end product of a process triggered by political convulsions in thirteenth-century Inner Asia. For several centuries before and after the Mongol irruption into West Asia, newly Islamicized Turks from Central Asia and the Iranian Plateau provided a ready supply of soldiers, both as slaves and as free men, for commanders such as Muhammad Bakhtiyar. Once within Bengal’s fertile delta, these men pushed on until stopped only by geographical barriers. Surrounded on the north and east by mountains, and to the south by the sea, Bengal was the terminus of a continentwide process of Turko-Mongol conquest and migration. It was, in short, a frontier zone.

In reality, Bengal in our period possessed not one but several frontiers, each moving generally from west to east. One of these was the political frontier, which defined the territories within which the Turks and their successors, the Bengal sultans and governors of the Mughal Empire, minted coins, garrisoned troops, and collected revenue. A second, the agrarian frontier, divided settled agricultural communities from the forest, Bengal’s natural state before humans attacked it with ax and plow. A third was the Islamic frontier, which divided Muslim from non-Muslim communities. A porous phenomenon, as much mental as territorial in nature, this last was the frontier that proved so fateful in 1947. Finally, all three frontiers were superimposed on a much older one, a frontier defined by the long-term eastward march of Sanskritic civilization in the Bengal delta. Characterized either by an egalitarian agrarian society organized around Buddhist monastic institutions or by a hierarchically ordered agrarian society presided over by Brahman priests, Sanskritic civilization in both its Buddhist and its Brahmanic forms had moved down the Gangetic Plain and into the Bengal delta many centuries before Muhammad Bakhtiyar’s coup of 1204.