A good collection of Plekhanov's early works, obviously significant for the development of Marxism in Russia.
In "Socialism and the Political Struggle," Plekhanov argues for the necessity of the political struggle and a socialist state against the dominant Narodnik trends at the time, similar to Lenin's early works. The same occurs with his book Our Differences, which could be read as a preface to Lenin's own The Development of Capitalism in Russia as both books combat the anti-Marxist economic theories idolizing the village commune, etc.
There is a lot of value in the articles "A New Champion of Autocracy" and "On the Sixtieth Anniversary of Hegel's Death" for their exposition of dialectical materialism and Marxist philosophical methodology, as well as reviewing Hegel's progressive contributions and mistakes. The Foreword and Notes to Engels' Ludwig Feuerbach is valuable for the same reason, the defect of all three works being a lack of exposition of the dialectical method itself and merely very basic introductions.
The Development of the Monist View of History is valuable for its historical review of the development of philosophy that led to Hegel and Marx, particularly the French materialists and Restoration historians, who are all too often ignored. There is basic exposition of the Hegelian dialectical method, and a review of the development of dialectical materialism out of Feuerbach and criticism of the Young Hegelians. Certain parts of the work could also be valuable for a critique of "Marxist" Humanist trends.