Ace Books
Founded in 1952, Ace was a major publisher of "double" novels, which featured two novels bound back-to-back. Ace Books published a variety of titles, but specialized in science fiction, mysteries, and westerns.
Avon
Founded in 1941, Avon was the first major competitor to Pocket Books. As Avon grew as a company, their mission centered on publishing original titles rather than reprints, and they published new works from a variety of genres. In the 1970s, Avon became a major publisher of historical romances; although romantic fiction was not a focus of some of the other top mass-market houses, for Avon, romances were one of the company's top earners.
Bantam
Founded in 1945, Bantam books went on to dominate the mass-market paperback industry in the 1960s and 1970s. Although they published a variety of works from mysteries to westerns, part of their dominance was due to their acquisition of reprint rights to "literary" paperback authors like Hemingway, Bradbury, and Steinbeck. They also were known for their cross-media promotion of their books alongside popular films, and for their "instant books," which were books based on current news that came out mere weeks (or sometimes days) after the event. Bantam also published many of the bestselling horror novels of the mid-70's resurgance of horror including The Exorcist, Jaws, and The Amityville Horror.
Ballantine
Ballantine Books was founded in 1952 by Ian and Betty Ballantine. Ian Ballantine's expertise in the mass-market paperback industry came, in part, from his previous work with Bantam Books in the years prior to Ballantine Books' founding. Ballantine was a major trend setter in the business, and started a fantasy publishing boom with their printing of The Hobbit. Although they were a major mass-market paperback publisher, Ballantine also guaranteed hard bound publications to authors who signed with them. Ballantine was aquired by Random House in the 1970's.
Dell
A major publisher of detective novels, westerns, romance, and science fiction, Dell was founded in 1943. Although it predominantly published a variety of types of genre fiction, Dell also published literary paperbacks by authors like Ernest Hemingway in addition to their genre staples. Like Avon, Dell began focusing on publishing original content rather than reprints in the early 1950's. Known for publishing the first major mass-market hit, Peyton Place by Grace Metalious, and for publishing Rosemary's Baby by Ira Levin.
Fawcett
Fawcett was known for publishing a wide variety of genre fiction, with specialties in suspense and romance. They also were a big name in purchasing reprint rights to hardcover bestsellers, and gained further notability both for the controversy around one of their early bestsellers (Women's Barracks by Tereska Torres) and for their mass-market reprint of The Godfather.
Popular Library
Founded in 1942, Popular Library focused mainly on mystery and detective novels. They had their greatest success with their printing of To Kill a Mockingbird by Harper Lee. Although the company had been independent throughout the mid-20th century, in the 1970s, Popular Library was acquired by Fawcett.
Pocket Books
Often credited with starting the "second paperback revolution" in the U.S., Pocket books was founded in 1939 by Robert DeGraff. The company prided itself on being easily transportable, cheap, and publishing a range of different genres and titles. Pocket books also popularized the strategy of relying on newspaper distribution networks for book sales.