Actually rather a collection of three essays than a monograph. One of them is solid, another one messy and he third one brilliant and groundbreaking. This most recent edition contains moreover a lot of additional material, the original conclusion, the conclusion of the 1972 edition (which now is the official one), essays by Lee Russell on various directors from the New Left Review, and an interview of Wollen by Russell. The importance of this book cannot be overrated as it reflects the efforts of the British Film Institute to transform Film Studies into a respectable academic discipline.
In the first chapter/ essay "Eisenstein's Aesthetics" Wollen sketches how the Soviet director developed his ideas on montage building on his discussions on the aesthetics of the theatre in late Czarist, early Soviet Russia, but integrating as well many aspect of his autodidactic studies of literature, the visual arts and Hegelian philosophy.
The second part "The Auteur Theory" is the least convincing one, as Wollen does not properly explain what auteur theory is, while he comes forth with a mass of brief description of he works of many, mostly Hollywood directors. In this respect less would have been more. Obviously I am not the only one who got his impression. Wollen seems to have received a lot of negative feedback due to which he elaborated on the topic again in the 1972 conclusion, where explains auteur theory more stringently as the attempt to analyze the common stylistic and thematic features in the work of one director.
Groundbreaking is the essay "The Semiology of the Cinema".* The main aspect is his adaption of the Peircean distinction between icon, index and symbol for the interpretation of the visual (and on screen sound) aspect of film, with the central aspect being that in film items represent themselves (icon) but also hint at (index) or represent (symbol) something else. Wollen draws as well attention to Lessing** who reflected on the overdetermination of the image as opposed to the underdetermination of language, and he proposes Propp and Russian/ Soviet formalism in general as most promising approach to understand narrative in film (which then became central for David Bordwell). This chapter/ essay is as well dedicated to the rehabilitaion of Eisenstein, for whom the symbolic and indexical were of such importance (although he did not call them like that) while film theory was dominated by ideas about the image representing "reality". It concludes with a praise of Godard (expanded in the 1971 conclusion) who according to him combined the in an ingenious manner the documentary and the representational aspect of film. Fun aspect of that chapter: Wollen alerts to the writings of an hitherto rather Italian scholar hitherto only known to insiders: Umberto Eco.
Russell's essays on different directors are relevant to the whole book as they are a kind of empirical underpinning of Wollen's essay on auteur theory, while the interview gives an interesting insight into the discussion of British intellectuals in the 1960s which were decisive for the development of Film Studies as a serious discipline, which by the way implied not to discard "Hollywood" and other commercial productions outright.
* A simplified version of it can be found in James Moncao's deservedly popular textbook How to read a Film.
** Yes, Gotthold Ephraim Lessing that German guy from the 18th century.