This question has been flagged as possibly being off topic with the reason "Request for an external resource - normally considered off topic on other sites."
In the case of this specific question, I am inclined to leave it, more because of the quality of the answer than of the question.
However, in general should they be regarded as on or off topic?
Rather accademic in spirit communities in the network have generally no problem with allowing study material / reference / data request questions. This is only an issue on sites that are too focused on the SE point of view at the expense of people who want to seriously learn and investigate the topic of the site. IMHO astronomy should rather endorse the more natural and liberal academic / scientific point of view and let people ask for study material / references / data etc if the question is reasonably narrowed down and well-defined.
So these in SE terminology so-called resource or "recommendation" questions should generally be regarded as on topic.
I think resource requests should be treated, for the most part, like list questions. If there is a way to produce a finite, best quality list of resources, then the question is good. If the only problem is that the list might increase over time, it might be worthwhile making a community wiki question. Otherwise, it should be closed.
These types of questions should be careful to cite the resources as fully as possible (title, author, publication data, etc.) so that it is possible to track down the resource (links to amazon pages or other webpages showing the resource are not sufficient because they are subject to link rot). For web-based resources, check the Wayback Machine and make sure they are cataloged before adding to the answer. This way if the pages go down, there is a backup reference.
