A "reader" differs from an introduction or a beginner's guide. A reader selects key passages from an author, and "brings the reader face-to-face with the writing itself in the company of an expert guide". Thus Ray Monk elucidates key passages of Wittgenstein.
It would seem that the Wittgenstein passages are well chosen, and well explained. As far as the "How to Read" books go, this one strikes a good balance of explanatory power and simplicity of style, and further points out some common mistakes in understanding Wittgenstein. A further strength is its plain explanation of the shift from the Wittgenstein of the Tractatus to the Wittgenstein of the Philosophical Investigations.
Monk expresses some strong views about Wittgenstein, and it would seem hard to tell whether he loves him or hates him. He quotes Wittgenstein's patron, Bertrand Russell, who considered that "the later Wittgenstein seems to have grown tired of serious thinking." This, he considers, may be "precisely right".
A major weakness of the book, I felt, was that Monk did too little to give one a sense of the wider significance of Wittgenstein's views -- or of their wider intent, if Monk should think that Wittgenstein had any. It is one thing to explain a passage in simple terms, another to explain its significance. So, for instance, Monk gives one little idea of the wider place of language games or of private language in the wider scheme of things.