Can a Bishop, forbid the reading of some books (or any kind of literature) to faithful under his rule, in such a way, that it would be sinful for the faithful to not follow his advice?
§ 1. The Church has the right of requiring that books that have not been recognized by her prior judgment not be published by the faithful,1 and that those published by anyone be prohibited for a just cause.2
1. "preventive censorship (praevia librorum censura)," for Catholics only
2. "vindicates to the Church the right of prohibiting any and all books which she considers objectionable," even for non-Catholics,
natural law grants "the right to control the reading of her children" to
Paternal as well as political authorities [who] have the natural right to ward off anything that may endanger the moral and physical welfare of their subjects, and to protect them against bad surroundings, company, literature, etc., in fact anything that is apt to cause insubordination, anarchy, or moral decay. The Church, being an autonomous society, with subjects for whom she is responsible within her own sphere cannot be destitute of the authority and power which enables her to keep her children uncontaminated and to safeguard them against the danger of perversion. Of all the dangers that imperil man's salvation bad literature is perhaps the most destructive. Hence the right to control the reading of her children cannot be denied the Church even from the purely natural point of vantage. Historical facts amply confirm the necessity of preventive censorship in Church and State.1
1. Cfr. the classical work of J. Hilgers, S. J., Der Index der verbotenen Bücher, 1904.
My dioceses' former Bishop Robert Morlino (RIP), at a homily about 15 years ago at St. John Vianney's in Janesville, WI talked about The DaVinci Code and how he "could ban it, but then you'd all go and read it".
So, in the USA, given our disobedient temperament, it seems that prudent to Bishops not do to a thing like merely ban a book, but just to let it die a natural death (like it did, nobody talks about The DaVinci Code any more and I feel bad for doing it).
However, the DaVinci code apparently was banned in Lebanon, of all places. Apparently out of sensitivity to Christians, not as a result of Bishops banning it explicitly.
Historically, there was an Index of banned book. In 1966, "the Index" of banned books was essentially ended:
To respond to the above-mentioned questions, this Sacred Congregation for the Doctrine of the Faith, after having asked the Holy Father, announces that the Index remains morally binding, in light of the demands of natural law, in so far as it admonishes the conscience of Christians to be on guard for those writings that can endanger faith and morals. But, at the same time, it no longer has the force of ecclesiastical law with the attached censure.
So, it would seem that a Bishop who could enforce a ban (i.e. whose ecclesial power extended into the secular world), might be still likely to make a ban, but since the prince of this world has been in charge for quite a while, bannable books are more easily accessible than ever and entirely up to individual conscience to avoid.
Viewing pornographic and illicit materials still carried the gravity of sin and should be avoided at all costs. This is mentioned in most examinations of conscience.