The prosecution is required to share any and all evidence it finds with the defense [s:2sk470hq]council[/s:2sk470hq] counsel of the accused. If the accused wrote letters, had books, or made postings, then it's pretty much assumed that the accused knows he did those things and there's no confidentiality involved (at least with regards towards him). As criminal trials are generally a mater of public record and most times there are also members of the public in the audience, then it's hard to say such evidence is confidential and that no one outside of the accused and the judicial system should know about its existence.
I would think that the only 'confidential' information would be that which needed to be kept out of the news to prevent alerting any accomplices (if there are any), or that which might corrupt other evidence or testimony.
Something could be made of how such information was made public, however. Anything released through official channels is by it's very nature, no longer 'confidential'. Information leaked by investigators or anyone involved with the investigation could be another matter.