In a deal that will unite the country’s two largest religion book publishers, HarperCollins, parent company of Zondervan, has reached an agreement to acquire Thomas Nelson for an undisclosed price. HC expects to close the purchase before the end of the year.

HC CEO Brian Murray said the publisher was attracted to Nelson because of its “great content and great authors.” He sees Nelson as being more broad based than Zondervan, pointing to Nelson lines in such areas as business and leadership. Nelson, he added, “is a leader in the inspirational market and we are always looking for good content.” Nelson has had one of the bestselling books of the year in Heaven Is for Real. The area where the two are in the most direct competition is the Bible category. Nelson and Zondervan are the dominant Bible publishers in the Christian market, and they license or own translations that compete head to head.

Murray said it was too early to discuss how Nelson will be integrated into HC, although he said HC will continue to maintain offices in Grand Rapids, Mich., home to Zondervan, as well as Nashville where Nelson is headquartered.

Mark Schoenwald, CEO of Nelson, which is owned by the private equity firm Kohlberg & Company, said having a publisher for a new owner made good strategic sense. Being part of HC will help Nelson speed its expansion in two key areas—digital and international, Schoenwald said. “We’ve made solid progress in digital, but we feel Harper has the infrastructure to accelerate our efforts,” Schoenwald said. “We feel they can be a multiplier for us.” Although Nelson just opened a new division in Mexico, Schoenwald said Nelson believes it has been under represented internationally and that HC’s global operation will help it improve its international sales quickly and efficiently.

Schoenwald said Nelson has had a good first half of fiscal 2012, with sales ahead of plan, even excluding Heaven Is for Real. Backlist nonfiction is doing well led by such established authors as Billy Graham and Max Lucado, while Christian fiction “has seen a lift” from e-book sales, he said. Children’s book sales “have been up and down,” Schoenwald said.

The purchase of Nelson is the largest in the book publishing industry this year, and the largest in several years. HC will also be Nelson’s third owner in 18 months. The company was originally acquired in 2006 by InterMedia in a $473 million deal. Kohlberg acquired a majority interest in Nelson in July 2010 as part of a major overhaul of Nelson’s capital structure.