Last week HarperCollins announced it was acquiring Thomas Nelson Publishers, a move that stunned many in the industry, since Harper already owns Nelson’s biggest competitor, Zondervan. The two are the largest Christian publishers in the world, and although they have definite differences—Zondervan is more narrowly focused on the Christian market and on materials for churches, while Nelson publishes more broadly, in categories such as business and personal finance—they are direct competitors in the Bible market.

Zondervan publishes the New International Version, the top-selling Bible translation; Nelson comes in third with its New King James Version. Completion of the deal is subject to approval by government regulators.

Amid rampant speculation about how Harper might structure its religion publishing operations if the deal closes, Erin Crum, v-p of corporate communications for Harper, told PW, “We don’t speculate about future plans.” Echoing the press materials sent out by Harper and Nelson, she said, “We plan to continue publishing both Zondervan and Thomas Nelson books and Bibles. They have complementary but different missions, and we will keep those missions intact.”

Harper has another religion-focused unit—its HarperOne division publishes religion books aimed at the general trade and academic markets as well as a small complement of Bibles, including the New American Bible Revised Edition for Catholics and other specialty Bibles in the New Revised Standard Version.

The Nelson acquisition would give HarperCollins a significant chunk of the religion trade and Bible markets, though how big that chunk would be is hard to quantify. The size of those markets is notoriously difficult to estimate; books with religion content span many genres, and publishers define what is a religion book in different ways. Sometimes they choose not to categorize their books as religion at all, in part because of shelving issues. BookScan does not collect data from religion-specialty stores; generally the acquisition is estimated to capture about 70% of known sales in the category.

AAP and BISG, who coproduced the new BookStats report, now agree on an estimate of $1.4 billion for the size of the religion book market in 2010. Reliable Bible sales numbers are even more difficult to come by (BookStats doesn’t break those out), but a conservative estimate by the Somersault Group, a consulting and publishing services agency formed by several Zondervan veterans, places unit sales at about 25 million annually.

Thomas Nelson has undergone other ownership changes in the past several years. The house was taken private again in 2006, in a $473 million buyout led by venture capital firm InterMedia Partners. In 2010, an investor group led by Kohlberg & Company acquired a majority of its stock; InterMedia Partners maintained a minority interest.

Zondervan, which was acquired by HarperCollins in 1988, is also seeing significant changes these days. The division will close its warehouse in Grand Rapids, Mich., in July 2012, when the takeover of fulfillment and shipping by R.R. Donnelley for all HarperCollins U.S. divisions, including Zondervan, is completed. Zondervan will give up its lease on the headquarters and warehouse complex and will be searching for a new location in the Grand Rapids area for its editorial and remaining back-office functions.

Nelson and Zondervan also have undergone recent leadership changes: in March, Moe Girkins stepped down as Zondervan CEO, a position she had held since 2008; the new CEO is industry veteran Scott MacDonald. Also this year, Mark Schoenwald took over as CEO at Thomas Nelson after Michael Hyatt stepped down; Schoenwald had been president and COO.