Look up a book on Amazon.com, and the first media review you see isn’t from a well-known book review outlet such as the New York Times or Washington Post but from Publishers Weekly. Scroll down, and chances are you’ll also find an opinion from Kirkus Reviews, Library Journal, or Booklist.

You’ve probably never read these magazines, even if you’ve seen their names on book jackets. But they’re helping determine what you read. Together, they make up the big four of book industry trade journals, aimed at publishing insiders: newspaper and magazine editors, bookstore and library book-buyers, literary agents, and film industry types scanning them for movie rights. Long important as behind-the-scenes power brokers, they became even more powerful in the 1990s, when online booksellers signed deals with them. (Barnes & Noble.com, like Amazon, has a deal with Publishers Weekly.) Their reviews—300 or so words of plot summary, context, and a quick verdict—influence which books get noticed, bought, and promoted in the media.