In my personal opinion, Books, study material, and reference request questions specific enough should be are allowed, as they have been on Theoretical Physics SE and still are on MathOverflow.
Some of the questions in the relevant tags are (or have even been) highly appreciated by these two communities. When looking at the main page of MathOverflow (and the same was true for Theoretical Physics SE), there was never any problem with such questions overwhelming the site or flooding down "normal" questions, as for example has often been claimed on Physics SE...
However, There are have been concerns about too much rep gained from such questions, but they can easily be alleviated by some kind of community wiki feature as discussed for example here or here. Properly implementing community wiki similar as it is available on SE might be complicated, but a cheaper work around would be to have a community wiki category which allows to zero the rep sililar as it has been done for meta.
Anyway, So as such questions are very useful for people who want to newly dig into a new (advanced) topic or learn about what is the current cutting edge concerning a specific issue, I think specific enough books/study material/reference questions should be are generally allowed.