Fr Z has a post Back in the day… forbidden books and seminarians reminds me of when I was a seminarian and some of the staff were just a little concerned about me having and passing around books by "that retrogressive, conservative theologian ". I remember being told by a priest who has since left, "if you are going to have them on you book shelf at least put them behind your ad usum books". Joseph Ratzinger was the author that caused concern! The ad usum titles were by Schillebeeckx and Häring, I was told it just after we had a seminary trip to listen to a lecture by Dr Hans Kung.
Thank God, my alma mater has changed out of all recognition.