Wittgenstein's Tractatus Logico-Philosophicus (http://en.wikipedia.org/wiki/Tractatus_Logico-Philosophicus) and Philosophical Investigations (http://en.wikipedia.org/wiki/Philosophical_Investigations). The former was a keystone of the Logical Positivist movement (and accepted by Bertrand Russel as LW's PhD thesis). The latter is its comprehensive dismantling (in note form only). Russell thought Wittgenstein had gone mad. We know otherwise :)