^I would look at it in two ways.
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Is the professor writing the book on their own time or being paid by the university to write the book? If the latter, the university should own it.
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As a practical matter, universities need to focus on cost cutting, where the potential savings is largest. It may be revenue from textbooks is too small for universities to worry about. If you accept the original premise of your post, that the primary driver of tuition increases is teaching salaries, then faculty compensation is too large to ignore.