Reviews frequently appear before a book can be bought. That is deliberate, and it depends on a version of the book that exists months before the finished edition.
Proof copies exist long before the book
Once a manuscript is edited and typeset, publishers print a limited run of unfinished copies. They carry uncorrected text, plain covers and a warning that quotations must be checked against the final edition.
These proofs go out well ahead of publication, sometimes six months or more for titles the publisher is investing in heavily.
Digital equivalents now circulate through review platforms, which widens distribution to reviewers a publisher would never have posted a printed copy to.
Printed proofs remain in use for the titles a publisher most wants noticed, because a physical object is harder to ignore than another file in a queue.
The trade has to decide before readers can
The earliest recipients are not the public. Booksellers, librarians, wholesalers and chain buyers commit to quantities months in advance, and they need something to judge.
Trade publications review on that same early schedule so those buyers have independent opinions before ordering closes.
By the time a general reader sees any coverage, the decisions that determine how many copies exist and where they sit in a shop have already been made.
Timing is coordinated on purpose
Publishers try to concentrate coverage around the on-sale date so that interest and availability coincide. A rave a month before anyone can buy the book wastes most of its effect.
Embargoes are the tool for this. A reviewer receives the book early on the understanding that nothing appears before an agreed date.
Embargoes hold most reliably where a reviewer values continued access, which is why they are broken more often around books with news value than around fiction.
Early readers shape the launch
Advance copies also go to readers with audiences, who post reactions as soon as the embargo allows. Those first impressions populate retail pages before publication day.
A book arriving with visible enthusiasm attached behaves differently in its first week from one arriving cold.
First-week performance then feeds the retail systems that decide what gets recommended, so early reaction has effects well beyond its own audience.
What the reader should know
An early review describes a text that may still change, though substantive changes at proof stage are rare and usually limited to corrections.
The more useful caution is about who received the proof. Advance copies go disproportionately to people already inclined towards the book, so the earliest reactions are the least representative ones.