Are there situations where taking an honorarium for trying a textbook on a class is acceptable behavior?
I think if you'd not seriously consider the text without the honorarium, that clinches a "don't do it". That is, external monetary motivations should not be allowed to color your professional judgement. And, given that the appearance of impropriety is itself a potential problem, even if the text is plausible, better to be able to later claim objectivity by not taking an honorarium/bribe...