eDiary and we can't wait to write in today's entry that today we answered a question about somebody else's diary. Actually, we never realized how much work it is to document daily life. And we have to say it's a little disappointing sometimes to re-read it and realize that our life is basically a series of meals interrupted by commentary. (Good meals, BTW).
Right, you had a question. If the author of the diary died before 1942, then nobody owns copyright. The work is in the public domain. If the author died after 1941, the diary would still be under copyright and republication as you describe would most likely be an infringement, and not a fair use.