On Saturday, April 19th 1817 , David Ricardo published The Principles of Political Economy and Taxation (price was 14s, and 700 copies were printed; later editions had 1000 copies each; my copy above is of the 3rd and definite edition published in 1821, and had at least two previous owners, a college and someone in Philly that signed it in 1901). Most comments on the book tend to emphasize things like rent theory and comparative advantage, but those are not central to the main point of the book (the Wikipedia entry is specially bad). The central question in the book, which follows Ricardo's Corn Essay of 1815 (An Essay On the Influence of a Low Price of Corn on the Profits of the Stock), is that there is an inverse relation between wages and profits, and, distribution is conflictive. That is the essence of the Ricardian theory, and the reason why Marx was a Ricardian, not a minor one as suggested by Samuelson though.
The inverse relation between wages and profits is what led Ricardo to criticize Adam Smith's problematic adding up theory of prices, and abandon the notion of labor commanded for the concept of labor incorporated (to keep the labor theory of value), which then led to the search for absolute value. The solution of the problem was elusive and was only provided by Piero Sraffa, the editor of his Works. For more read this entry on the Labor Theory of Value and this one on Sraffa's Standard Commodity.