When conducting interviews in the U.S., I'm very careful when asking soft questions like that.

If a candidate's answer included a book that touched on a topic from some legally-protected class (e.g., race, religion, or sexuality) and that person didn't get hired, it could be one more piece of ammunition in a hiring-discrimination lawsuit. (I do wonder if I'm being overly cautious on this.)

I'd probably rework the question to something like, "Have you found any good books, websites, etc. that you've found useful for growing the skills this position requires?"