OVERALL, looks like there are 3 big K12 textbook publishers....HMH, Pearson & McGraw Hill.(Lots of others turn out to just be divisions of these three.)

WIKIPEDIA says As of January 2009, the four largest college textbook publishers in the United States were:

• Pearson Education (including such imprints as Addison-Wesley and Prentice Hall)
• Cengage Learning (formerly Thomson Learning) NO K12
  
• McGraw-Hill
• Houghton Mifflin (including Harcourt)

Other (NO K12)US textbook publishers include:

• John Wiley & Sons
  
• Jones and Bartlett Publishers
• F. A. Davis Company
• W. W. Norton & Company
• SAGE Publications

K-12 EDUCATIONAL PUBLISHERS

#1 Houghton Mifflin Harcourt
includes Holt McDougal division
includes Saxon, provides K12 math & science textbooks and online/homeschool programs.
#2&3 are...
McGraw Hill
owns Glencoe, which has textbooks and an "e-solutions" business
Pearson
owns Prentice Hall & Addison Wesley

Thompson educational (bought by Apax Partners pvt equity for $7.7B in 2007)???Not sure what happened to them or if K12. Part of Cengage, the big non-K12 publisher?

List of K12 TEXTBOOK PUBLISHERS
http://dir.yahoo.com/Business_and_Economy/Shopping_and_Services/Publishers/Education/Textbooks/K_12/


THE REST BELOW JUST MISCELLANEOUS TIDBITS

Houghton Mifflin Riverdeep Group formed 2006 when Irish company Riverdeep (Dublin) combined with Houghton Mifflin (Boston) in $3.4B deal. In 2007, it bought Harcourt educational business for $4B from Elsevier. Now largest

Holt McDougal, a division of Houghton Mifflin Harcourt Publishing Company.

Two of the most trusted names in grades 6-12 educational publishing - Holt, Rinehart and Winston and McDougal Littell - have joined to become Holt McDougal, a division of Houghton Mifflin Harcourt Publishing Company.

http://holtmcdougal.hmhco.com/hm/home.htm;jsessionid=E5CA7393293B3CCAD359F9154A815659.cz-app-wk1

Pearson

Pearson is the world's leading PreK–20 educational publishing company, dedicated to working with educators to change the way America thinks.

Prentice Hall, Inc. | A Pearson Education Company | Upper Saddle River, New Jersey 07458

Glencoe, a McGraw Hill Company glencoe.com has e-solutions business.

http://www.nytimes.com/2007/07/23/business/media/23publish.html article on textbook company acquisitions in 2006-07

2004 That's just the latest in a string of ambitious notions that have helped O'Reilly turn his company, O'Reilly Media, into the nation's second-largest tech book publisher. This fall more than 100 computer science professors nationwide will do the heretofore impossible: Through a new service called SafariU--a partnership between O'Reilly and Pearson Technology Group, a division of the $7.1 billion Pearson publishing juggernaut--they'll assign all-in-one textbooks, customized online to their exact specs. From a database of more than 5,000 O'Reilly and Pearson titles and articles, profs can click and drag anything from a few paragraphs to multiple chapters into their course syllabus. Then they can upload whatever outside material they need--articles, exercises, links, exams--and add it to the mix. Once it's digitally assembled, the text is instantly online; a print shop in Somerville, Mass., then cranks out hard copies to be shipped to the college bookstore.

Wiley: Our portfolio of global brands includes For Dummies, Frommer's, Betty Crocker, Pillsbury, CliffsNotes, Webster's New World, J.K. Lasser, Jossey-Bass, Pfeiffer, and Sybex.

Wolters Kluwer educational (bought by Bridgeport Capital for $1B in 2007) K12 none