Recently the private diary of Rav Kook was published; diaries of other Rabbis have also been published posthumously. Is there any halachic problem with publishing these documents, and, if there is, is there any halachic problem with reading them once they are published?
I'm going with the notion that it's unethical and thievery. And even if the person who stole it is a theif, it will still at least be muttar for the rest of us to learn, and even a zchus for the theif. See the Shach in Sh'a Choshen Mishpat siman 292 siff kattan 35. He quotes a Tosefta in the 7th perek of Bava Kama if someone steals his friends torah (lets assume that means chidush, an intangible idea) and goes and teaches it, even though he is called a theif, he is zocheh it for himself etc, and he will be made a parneis hatzibur and he is mizakeh the public and zocheh for himself etc.
I would also add the famous story where the Arizal comanded Reb Chaim Vital to never share the knowledge he tought him with anyone. Reb Chaim kept all the chidushim safely under lock and key. Till one day Reb Chaim fell ill and people bribed one of his relatives to allow them to view and copy the papers. And from this incident one of the most respected and acclaimed teachers of the Kabbala had his ideas spread to the world.