In theory, reviewing a book is fairly straightforward. A reviewer reads the book, they may make notes or bookmark pages for quotes while reading, but they read the whole book. The book may be put aside for a while so the reviewer can think over what they’ve read or plan out what they want to say in their review. A review is drafted. The reviewer may put that aside and come back to it to check they’ve covered all the points they wanted to mention. The reviewer may re-read the book to check they’ve not missed something. The review is copy-edited, quotes checked and edited for publication. That may seem long-winded but essentially the book is read, the reviewer forms a draft, their review is written.
A review is more than an opinion. Yes, it contains opinion(s). But it also acts as a guide: it gives the reviewer reader enough information to determine whether they want to read the book under review. Anyone can say they liked or didn’t like a book but a review needs to say why. Anyone can express an opinion on social media that a thriller wasn’t that thrilling but a reviewer needs to be clear about why the thriller fell flat. Anyone can write off a poetry collection as doggerel but a reviewer will back their opinion with examples and explanations.
A reviewer has to read the whole book. A publisher or agent, particularly an experienced one, may only bother with the first page or even paragraph before deciding they don’t need to read further. A reviewer may be tempted to give up after the first chapter, page or paragraph, but they need to read on. A general reader is free to stop reading whenever they feel they’ve had enough. While it’s a reasonable expectation that the publisher or agent won’t talk about submissions outside of their working environment, a general reader is free to tell their social circle about their disappointment in a book.
A reviewer has to read the whole book. Even if the first chapter sets the entire tone and there are no surprises mid-way though. Even if the wished-for plot twist never materialises – you’d have to read the book to know it didn’t. Even if you’ve read the last twenty books by this particular author and know they never deviate from their formulaic plots. Even if you’re familiar with the writer’s work from extensive reading of poetry magazines, know the type of poems their publisher favours and can guess what the contents will be like before opening the collection, you still need to read it. If only because that plot twist just might happen in the final chapter, that final poem might be the stunning gem that outshines all others and if your review doesn’t mention it, you’ll look like the idiot.