Our page on what is on-topic in the Help Centre is very much a work in progress (to put it mildly). This means we're not really providing much guidance for new users on the site.
It seems that hardware recommendation questions are on-topic, provided enough detail is provided to allow for meaningful answers (e.g. intended use-case, budget, etc.)
So should we allow requests for books and/or articles?
I am reliably informed that several sites do allow reference requests, while others do not. This is something that each Stack Exchange site needs to decide for itself.
If reference requests are to be allowed, should we apply any constraints to the scope of such questions?