The trial against Apple is scheduled to start in June 2013.
The US Department of Justice (US DOJ) has reached a settlement with Holtzbrinck Publishers over claims of collaboration to increase the price of e-books.
In April 2012, the US DOJ sued five publishers and Apple, claiming that they conspired to inflate e-book prices.
After the settlement, Apple remains the lone litigant in the case after Macmillan became the last publisher to sign a settlement deal.
The department said it will continue to litigate against Apple to prevent continued restrictions on price competition. Apple's trial is scheduled to start in June this year.
US DOJ's antitrust division chief of staff and counsel, Jamillia Ferris, said: "As a result of today's settlement, Macmillan has agreed to immediately allow retailers to lower the prices consumers pay for Macmillan's e-books."
"Just as consumers are already paying lower prices for the e-book versions of many of Hachette's, HarperCollins' and Simon & Schuster's new releases and best sellers, we expect the prices of many of Macmillan's e-books will also decline," Ferris added.
Under the proposed settlement, Macmillan will immediately lift restrictions it has imposed on discounting and other promotions by e-book retailers. The company will be prohibited until December 2014 from entering into new agreements with similar restrictions.
The DOJ said the proposed settlement was filed in the US District Court for the Southern District of New York and if approved by the court, the settlement will resolve the department's competitive concerns involving Macmillan.
The latest settlement follows the US DOJ"s settlement with book publisher Penguin Group (USA) in 2012 along with Hachette Books, HarperCollins Publishers and Simon & Schuster.