Sartre's Critique volume 2 follows up many of the issues raised in volume 1, exploring the dialectical intelligibility of history and the relationship between praxis, inertia and group-formation. Whereas volume 1 concentrates mainly on the different kinds of groups which can emerge from praxis, volume 2 concentrates mainly on the history of the USSR, treated as an example of the interaction of praxis and matter (inertia). Through this example, and broader discussions of the philosophy of history and the relation of agencies in conflict, Sartre develops a dualistic philosophy where historical outcomes rely on the interaction of two distinct fields, the field of agency associated with phenomenological meaning-construction and subjectivity, and the field of inertia associated with the deviation or alienation of praxis by matter.
This is an important work for students of critical theory and philosophy, including those interested in the development of western Marxism, people exploring the possibilities for Marxist theory, students of phenomenology and meaning-formation, and scholars interested in debates between poststructuralism and Marxism about meaning and discourse. Important and interesting, though not necessarily adequate; the methodological dualism leaves the application of the method dependent on intuitive judgements, and the application to the USSR in particular is weakened by a persistent bias towards the Bolshevik and later Stalinist way of viewing the situation. As a result, the account is open to objections that anti-regime agency is downplayed or dehumanized and that Sartre wrongly assumes a continuity between his own humanistic Marxism and the official ideology of Stalinism. Since Sartre is also not saying anything very original about the USSR, I'd say the book is of more interest as a text in philosophy than a text in Soviet studies.
I would just like to point out that, while Sartre's monumental Critique of Dialectical Reason is normally published in two volumes, the 1982 paperback edition comprises the whole work in a single volume.