Book publishing made history early Monday morning, as Random House and Penguin announced the completion of their long-awaited merger: the creation of a global behemoth with $3.9 billion in sales and more than 10,000 employees.

Billing itself as "the first truly global publishing company," Penguin Random House will have approximately 25% share of both the U.S. and global markets for general interest books.

Analysts have described the merger as a response to a shrinking bookselling environment and the growing power of Amazon. A powerhouse publisher, it's believed, could have more weight with the e-tailer, which is known to use brass-knuckled bargaining tactics, like removing the "Buy" buttons from a publisher's books, to get what it wants.

Random House parent company Bertelsmann owns 53% of the new venture. The German media corporation emphasized the firepower the new company will have as it adapts to a changing industry.

"Together, we can and will invest on a much larger scale than separately in diverse content, author development and support, the publishing talent, the entire spectrum of physical and digital book acquisitions, production, marketing, and distribution, and also in fast-growing markets of the future," said Bertelsmann Chief Executive Thomas Rabe in a statement.

Penguin owner Pearson holds 47% of the new company, which will publish more than 15,000 titles from nearly 250 imprints and houses across five continents.

Though Penguin Random House may have a giant footprint, the company is taking pains to tread lightly. Chief Executive Markus Dohle—formerly CEO of Random House—sent out a letter to booksellers Monday morning promising business as usual, with investment in new cutting-edge facilities down the road.

The new company will also continue doing business in the current locations where the two publishers have long-term leases: Random House at 1745 Broadway and Penguin at 375 Hudson. And Random House divisions will continue to bid against each other—and against Penguin—in auctions to sign authors as long as an outside publisher is also bidding.

Penguin will continue to offer a "house bid," rather than have its imprints bid against each other.

The companies are known for having distinctly different cultures: Penguin is regarded as more informal and collegial, while Random House offers higher pay and better vacation and benefits packages. A spokesman said that for now the companies will continue on their separate tracks.

Though analysts have spoken about reduced back office costs as one benefit of the deal, the spokesman insisted that there were no plans for cuts, pointing out that warehouses at both companies, for example, were working "pretty much at capacity."

But observers say that the business-as-usual approach can only last so long before cost-cutting and layoffs begin.

"There simply has to be a lot of overlapping functions," said Robert Broadwater, managing director of Broadwater & Assoc., an investment banking firm that specializes in publishing. "That's one of the reasons you do these things."

Agents are also eying the new entity with some caution, at least for their most high-profile titles. Though the different parts of Penguin Random House will bid against each other, there may be times when no outside bidder emerges to drive up the price.

"Agents are going to have to be strategic," said Scott Waxman, co-founder of the Waxman Leavell Literary Agency. He expected that he would make more use of soliciting preemptive offers from a single buyer in an effort to get the highest price for a project.

Small publishers were also concerned about a behemoth tilting the literary playing field.

Dennis Johnson, co-publisher of Brooklyn-based Melville House, estimates that Penguin Random House will control more than 50% of the market for literary fiction. That will give the company outsized power with hard-pressed independent bookstores, where shelf space is at a premium and bills are often paid in order of the publisher's bargaining power, he said.

"That's going to make it harder for me to get my books into bookstores," Mr. Johnson said.

While some booksellers have voiced concerns about having a single account make up such a large portion of their business, others see Penguin Random House as a little like the cavalry coming to the rescue. They like the idea of a stronger publisher that wants to invest in the future of physical books, and that can do a better job of standing up to Amazon, which they believe wants to destroy the physical bookstore business.

"The stronger they are, the stronger an ally we have," said Cathy Langer, lead buyer at Denver's Tattered Cover, a leading independent bookstore.