“IT’S official,” Alfred A. Knopf Sr. tweeted last week. “We’re now #PenguinRandomHouse.”

Mr. Knopf — or rather his ghostly avatar, the actual publisher having sold his namesake firm to Random House in 1960, died in 1984 and rolled over many times since — was celebrating the largest book-publishing merger in history.

The merger, announced last October and completed on July 1 after regulatory approval, shrinks the Big Six, which publish about two-thirds of books in the United States, down to the Big Five. HarperCollins has reportedly been flirting with Simon & Schuster, which would take it down to four. (The others are Hachette and Macmillan.)

The creation of Penguin Random House (“the world’s first truly global trade book publishing company”) is partly a response to unprecedented pressures on these “legacy” publishers — especially from Amazon, which came out on the winning end of an antitrust lawsuit over the setting of e-book prices. It is also a way to gain leverage and capital in an industry that has been turned upside down. This endgame may be inevitable, but its consequences can’t be ignored.

Consolidation carries costs you won’t find on a price sticker. Dozens of formerly independent firms have been folded into this conglomerate: not just Anchor, Doubleday, Dutton, Knopf, Pantheon, G. P. Putnam’s Sons and Viking, which still wield significant resources, but also storied names like Jonathan Cape, Fawcett, Grosset & Dunlap, and Jeremy P. Tarcher. Many of these have been reduced to mere imprints, brands stamped on a book’s title page, though every good imprint bears the faint mark of a bygone firm with its own mission and sensibility.