Is it mortal sin to read a book which one knows to be on the Index?

Yes, unless he secures special permission for grave reasons to read it. The only books named in the Index are those which are sinful and immoral of their very nature, or which have been proved to give an entirely false and even blasphemous view of Christian doctrine.

Radio Replies Volume 1 by Rev. Dr. Leslie Rumble MSC and Rev. Charles Mortimer Carty