Materialism and Empirio-criticism, 1908
Lenin's defence of philosophical materialism against a 'fad' resulting from the unusual results of modern physics.
See Ilyenkov's Leninist
Dialectics and the Metaphysics of Positivism for a
defence of this work of Lenin's and Teresa
Ebert who turns these ideas against modern bourgeois
philosophy. Many of Western Marxists have, however,
criticised this book. See for example, Marx at the Millennium by Cyril Smith.
For examples of the writers Lenin is criticising, see Ernst Mach, Poincare
and Helmholtz.
Three Sources & Three Component parts of Marxism, 1913
Brief and highly readable summary of marxism.
Conspectus of Hegel’s Science of Logic, 1914
Lenin's annotations on Hegel's Logic, interspersed with Lenin's aphorisms and observations on Hegel's dialectics.
You can read Hegel's Shorter Logic and Science of Logic to get an idea of what Lenin's annotations are about. For comments on these notebooks see An Introduction to The Philosophical Notebooks of Lenin by Cliff Slaughter (1962) and Raya Dunayevskaya (1973) and C L R James's Notes on Dialectics, including his criticisms of Lenin's reading of Hegel.
On the Significance of Militant Materialism, 1922
Lenin's appeal for a 'united front' between communists and natural scientists against the influence of religion and in defence of philosophical materialism.