The book reviewer has a substantial responsibility: her piece will be, more often than not, the only contact the reader has with a particular book. You need both to present an author's arguments fairly, and to make your case on why a book is relevant - or is not. Sometimes book reviews degenerate into opinion pieces, as the writer takes a book as a convenient excuse to make her cause for or against a particular policy / set of ideas / scholar. These are typically bad book reviews. But bad book reviews are also those in which the reviewers aim to present all the major arguments advanced by the reviewed author. This is a daunting and almost impossible task, if the book has any substance. The reviewer needs to provide her reader with a glimpse, the scent of the arguments; she needs not explain or demolish all the author's points; she needs to do a good service to the reader, providing some basic information on the reviewed book (whom was it written for? is the language accessible? should it be read cover to cover or can it be consulted from time to time? what kind of debates is it engaged with?) but also offering a sense of the style, tone, and argument of the book. You should not finish a book review thinking that you agree or you don't agree with the author's thesis. You should finish a book review having realized whether you do or don't want to get engaged with the author's thesis, i.e. to read the reviewed book.
I do, however, think it’s a good idea to take the key elements of reviews and use them to become a better writer in your next work. For example, if you have a good chunk of reviews that say the story dragged in the first one-hundred pages, make sure your next novel starts off with a faster pace that carries it through to the end. Bad book reviews don’t necessarily have to be a bad thing.
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