A judge took some letters of mine. Can he release them to anyone else but me?

Whether a judge would share letters that he obtained in your case depends on the purpose of the letters and why the judge kept them. For example, sometimes people write letters to a judge on behalf of a defendant at sentencing and those letters are kept in the file, but the court usually does not publish or share them. If a judge accepted letters you wrote as evidence in a case, then the letters may become part of the record in your case and the letters could be made public. A judge can't discuss the details of an ongoing case, but again, if your case is public record then the letters could be made public. If you are worried about the letters becoming a part of the record then you can file a petition to have them sealed.