"It's fully legal to read and publish the private letters and intimate diaries of historical figures"
Only if you don't violate the copyright of the dead person - which, depending on the country, can last 50 to 90+ years after the person's death. Thus to publish you'd have to get the right to do so from said person's estate.
There are many instances where private correspondence, etc. has been banned from publication/the public domain for this reason.
This goes in with the legal principle that these dead people don't have rights, but their surviving relatives do - if whoever inherited those letters and the rights to them decides to sell or publish them, the dead person's privacy is not grounds to prevent that.
Only if you don't violate the copyright of the dead person - which, depending on the country, can last 50 to 90+ years after the person's death. Thus to publish you'd have to get the right to do so from said person's estate.
There are many instances where private correspondence, etc. has been banned from publication/the public domain for this reason.