For nearly $1.8 billion, the textbook-publishing giant has accepted a buyout from Riverdeep Holdings, an Irish educational software company. An new company, HM Rivergroup will emerge from the merger, with Riverdeep assuming $1.61 billion of debt. The fourth-largest textbook publisher in the United States, Houghton Mifflin, with an annual revenue of more than $1 billion, was founded in 1832, it was home to such authors as Emerson, Thoreau, and Tolkien.