In 1836 the British commenced a policy of importing labourers from India to do the work of the former slaves, many of whom, as the book makes clear, no longer worked on the plantations. These labourers undercut the wages of the former slaves and were perceived to work harder. The black population was not happy about the consequent loss of potential jobs, which is why it was unwise for Mr Mason to talk about the scheme in the hearing of his servants.
Between 1836 and 1917, approximately half a million Indians were brought into the British West Indies. Today, Indo-Caribbeans form the largest ethnic group in Trinidad & Tobago, and the second largest in Jamaica. Nobel and Booker Prize winner V.S. Naipaul is a Trinidadian of Indian descent.
Coolie means an unskilled Asian labourer, and is now considered a racial slur. For an informed, if rather un-PC, account, see Edgar L. Erickson's 'The Introduction of East Indian Coolies into the British West Indies' (1934).