Here's my suggestion: change the incentives. Textbooks currently run pretty afoul of a third party payer problem. The textbooks are chosen by professors employed to teach, but purchased by students (who, in turn, often finance the whole thing with loans). There is little incentive then to use open source textbooks (you're not paying for them) so there is little demand so there is little supply.
What if a state simply mandated that universities and schools pick up the tab for textbooks that are required for courses? The schools could be allowed to alter tuition to pay for the books -- but the changes in tuition have to be flat or by department. Very quickly, schools would have great incentives to watch the prices of books closely. And academics just may see rewards for contributing to open source textbooks that could be reprinted by the college itself for free.