I'm interested in what specific differences in Lenin's writing might address what he perceived to be the shortcomings of Marx's work. Wikipedia is weak on this topic.
In my reading of Lenin, he very rarely criticizes Marx. He is much more interested in developing the dialectical concepts of Marx (ie. Speaking to the development of capitalism, the worker's struggle , and the tactics of the of socialist movements. ). That said , his major theoretical developments was to challenge the overly mechanical interpretation of historical development by contemporary Marxists. Marx had divided historical development into 5 successive stages (primarive communism, imperial city state, feudalism, capitalism, and communism). Many Marxists held out that different regions were at different stages of economic development and that it was necessary for each region to proceed through each stage in order. Lenin's disagreement with this was important in the development of the Bolshevik movement, as most socialists agreed that Russia was in the feudalist stage and needed to develop capitalism (see primative accumulation) before developing socialist policies.
One of his other noteworthy innovations was the concept of the vanguard. He believed that without an intellectual class distinct from the working masses that the highest level of worker organization possible was that of the trade union. The vanguard was necessary to organize and instruct the uneducated and distracted worker.
As noted earlier his theory on capitalist development are outlined in Imperialism: The Highest stage of Capitalism, his tactical thoughts on organizing are best found in Left Wing Communism: An Infantile Disorder and What Is To Be Done. Particularly relevant to this post is Karl Marx: A Brief Biological Sketch with an Exposition on Marxism. All can be found at http://www.marxists.org/archive/lenin/works/sw/index.htm