Wittgenstein's Tractatus - the only book he actually published within his lifetime - was an immensely important work which changed the direction of philosophy in the first half of the twentieth century.
Higlighting the importance of the nature of language in philosophy and the problematic nature of metaphysics, it strongly influenced on the work of Russell, the Vienna Circle an A.
J. Ayer. The posthumous publication of the Philosophical Investigations initially led many people to think that Wittgenstein had turned his back on the ideas expressed in the Tractatus and for a time the earlier work was neglected in favour of the later ideas.
However, more recently many scholars have begun to argue for a continuity between the early and the later Wittgenstein and it is increasingly felt that an understanding of the ideas in the Tractatus is essential to fully grasp remarkable later work.