Is it fair to blame authors for exorbitant book costs? Summary (TL;DR): The following's biased; so are there any counterarguments? Please beware that this question concerns only authors who are professors and who also write textbooks.
Are authors truly helping students as much as possible? For example, are textbooks edited and revised every two years out of greed? Certain subjects tied to real-life reforms (like law) may require such turnover, but what of Economics (as below) or Calculus?

[Supplementary Context] [Source:] Ray Lopez December 16, 2014 at 12:22 pm
LOL from the comments section of the npr post:

Mankiw is the worst sort of economist. He’s a blatant liar. Why does his book cost $320? Because that’s how much the publisher, Cengage, has determined the market will bear. Why does he not advocate reducing the price of the book to make it affordable to more students around the world? Because he wants to make as much money for himself as possible. (This is also why pharmaceutical companies often price their newest drugs at outrageously high prices; to maximise profits in the short term, rather than to deliver the greatest public health good.) Mankiw does not care about the impact of his ridiculously priced book on the purchasers of his product, and he papers over any twinge of guilt he might otherwise experience by convincing himself that his personal brilliance is worth the extra money. Congratulations, professor, for playing your little part in the grand national pastime of burying America’s college students in debt.

(no sympathy from me as he dissed outrageously priced on-patent drugs, but it’s telling how people think education is a public good. Nothing can be further from the truth. If you can’t afford an Ivy League education you have no business applying to Harvard…)
Short answer: both yes and no.

Partially, no they are not responsible, because some of it is a matter of costs and markets.  You're never going to see textbooks offered for the same price-per-page as romance novels, because the cost of creating and curating the textbook is so different.  So the base price of textbooks is expected to be significantly higher.

At the same time, however, authors have a lot of choice which publisher to work with, and some publishers are much more predatory in their pricing than others.  Most significantly, there are a number of university and society presses that can give a book quite good promotion and also a reasonable price.  Consider, for example, the MIT Press, and these widely used books: the CLR(S) Algorithms textbook ($70) and the SICP programming textbook ($49) and also free.  Moreover, academic publishing is a seller's market, unlike fiction publishing: in my experience, big academic publishers are always soliciting for authors to create books for them, and so there is space for an author to select (if not necessarily negotiate).

I would say, than, that any author who has courage of their convictions with regards to education can make a publishing choice that keeps their textbook well under $100 US.  Doing otherwise demonstrates either that the person is not caring about student costs or else that they think like an economist.