Houghton Mifflin Company, the US-based publisher of educational books/materials, has entered into an agreement to be acquired by Ireland's HM Rivergroup PLC for about $3.4 billion ($1.8 billion in cash, and the assumption of some $1.6 billion in debt).  In addition to purchasing Houghton Mifflin, the newly established HM Rivergroup will also acquire Riverdeep Holdings Ltd, a US publisher of interactive educational and personal publishing products, in a share-for-share swap valuing Riverdeep at upwards of $1.2 billion, including the assumption of debt.  Upon completion of the acquisitions, which is expected to happen before the end of 2006, HM Rivergroup will change its name to Houghton Mifflin Riverdeep Group PLC.