Q, when I was working on my masters in language arts instruction, I had a course titled Content Area Reading and this just happened to be during the fall of 2000 with the Bush/Gore campaign/election (and recounts of hanging chads).
One of the assignments was to analyze a textbook – its layout, print/font, relevancy and currency of photos, graphics, whether and how it was organized for optimal learning, etc. (That was the easy part.)
The other part of the assignment was to research the writers/editors/contributors, identify the college/university where they were employed, the courses they taught/teach, noteworthy research, papers, etc., and most importantly: Their political affiliations and leanings.
I chose an American History textbook and definitely observed liberal bias and leanings even then. I remember specifically the chapter about Reconstruction and how the word,"radical" (as in the Radical Republicans), was conveyed in a negative connotation by failing to elaborate how what was radical was their idea to give each freed slave 40 acres and a mule, which the Democrats opposed. A non-discerning reader would come away from that passage with the notion that Republicans were/are crazy "radicals".
Even more eye-opening, though, was to have learned state textbook adoption committees and how textbooks were/are approved by only a handful of state boards of ed.
I remember the professor telling us that only seven states had textbook adoption committees and the other 43 just followed their lead. In other words, the 43 states without textbook adoption committees simply trusted that the seven states with textbook adoption committees had done a thorough job, and would simply approve the textbooks for adoption in their states.
Now keep in mind this was during fall 2000 during the Bush/Gore election. The red/blue state maps were a constant on t.v., even during the recount.
But here's the clincher:
All seven states that had textbook adoption committees on the state board of ed were BLUE