Colletti is best know for his work that tried to undermine the link between Marxian and Hegelian dialectics and his argument that Marx's true philosophical ancestor was Kant.
For Colletti, Hegel's work was a return to pre-Kantian metaphysics. In fact Colletti assimliates Hegel to Spinoza and gives Marx over to Kant. He argues that the real is
always something more than the concept and this 'something more' that prevents
the reduction of being to thought. (cf Smith p 153). This work (Marxism
and Hegel) was to win praise, albeit with some reservations, from the French
Marxist, Louis Althusser for its rigour and profundity (cf: Reading Capital p 75). Mario Tronti
(one of founders of Operaismo) commended the early Colletti for his resolute
stance that anti-dialectical philosophy need not imply reformist politics.
Bibliography
Marxism and Hegel (1979)
Introduction to Marx's Early Writings available here
From Rousseau to Lenin
Twilight Of Ideology (1980)
Secondary
A.A.Smith - Hegelianism and Marxism: a reply to Lucio Colleti - Sicence and Society Vol L No2 Summer 1986
That Tronti piece - (only in Italian)
Guardian Obituary
From Marx to Berlusconi: Lucio Colletti and the Struggle for Scientific Marxism Rethinking Marxism Volume 22, Issue 1 January 2010, p. 148 - 156
A Political and Philosophical Interview (pdf) Perry Anderson Interviews Lucio Colletti for the New Left Review (1974)