A number of books have criticized multiculturalism, but even if you've already read a bunch of them, Salim Mansur's Delectable Lie: A Liberal Repudiation of Multiculturalism is still very much worth your attention. Mansur, a syndicated columnist who teaches political science at the University of Western Ontario and whose previous books include Islam's Predicament: Perspectives of a Dissident Muslim, approaches multiculturalism from the distinctive viewpoint of a naturalized Canadian citizen who is also a secular Muslim born on the Indian subcontinent. At once very knowledgeable about the history of multiculturalism and richly steeped in the long tradition of Western ideas about individual liberty (of which he rightly recognizes multiculturalism as a profound philosophical violation), Mansur is also a highly effective polemicist. Although awash in learned references to thinkers ranging from Plato and Aristotle to Karl Popper and Friedrich Hayek, Mansur's book is eminently accessible, and should be of interest to any reader who is concerned about the threat that multiculturalism poses to the Western heritage of freedom.
It's significant that Mansur is Canadian, because Canada, as he puts it, was "the first major democracy to experiment with designing a society on the basis of multiculturalism." He recounts the origins of this policy, which took shape largely as a response to growing pressure for Quebec's independence (or, at the very least, for radical revision of its position in the Canadian confederation). This pressure led to Prime Minister Lester Pearson's 1963 establishment of a Royal Commission on Bilingualism and Biculturalism, which in turn eventuated in the formulation of an official multicultural policy in 1968 by the government of Pierre Trudeau. Thus began Canada's shift from a liberal democratic society that supported individual rights to a nation that placed the rights of the group above those of the individual – a process that reached its culmination under Brian Mulroney, during whose prime ministership, Mansur maintains, "Canada became the first western liberal democracy to adopt multiculturalism as the defining characteristic of the country."