If you're interested in learning more, here are some sources that will provide a good start.
Kaldor, Nicholas. An Expenditure Tax. London: Allen & Unwin, 1955. An early and brilliant exposition of the nature of and case for a cash-flow, or postpaid, consumption tax.
Mill, John Stuart. Principles of Political Economy. 1848. Reprint, edited by J. Riley, Oxford: Oxford University Press, 1994. Mill was perhaps the greatest writer in the tradition of political economists who freely mixed philosophy, economics, and politics. Here he articulates the case against income taxation and sets out the double-tax criticism.
Musgrave, Richard. The Theory of Public Finance: A Study in Public Economy. New York: McGraw-Hill, 1959. One of the seminal texts in public finance by a major economist.
Simons, Henry Calvert. Federal Tax Reform. Chicago: University of Chicago Press, 1950. Also Personal Income Taxation: The Definition of Income as a Problem of Fiscal Policy. Chicago: University of Chicago Press, 1938. Simons was the major conceptual thinker behind the income tax, and his two books retain much historic interest.
Smith, Adam. An Inquiry into the Nature and Causes of the Wealth of Nations. 1776. Reprint, edited by Edwin Cannan, Modern Library, New York: Random House, 1965. This remains a wonderful volume. Book 5 considers taxes. Smith is important not only as an early pioneer of free market theory, but also for his general method and his approach to questions of public policy. Smith and his colleagues of the Scottish Enlightenment, such as David Hume, were skeptical of overly idealistic theory and made a point of paying careful attention to human nature and practical politics. Smith's observations on taxation reflect these traits.