I have found myself using Math.SE for finding books on various subjects.
The way I am doing this is writing the subject followed by the word book, or by filtering the search results with the reference-request tag.
For example, just this week I have searched "Probability book" to get to some books that were recommended in the past.
This method also brings back results like questions from the books (exercises).
Do we need a tag for books (or book-recommendations) tag?
P.S: I don't know if this feature exist, but I would like to see it [silly me, forgot to ask for the feature!]: Can it be a sub-tag of reference request?