I have a question

[michelemicks]

Who do you recommend to learn Forex from?

[HENARRY JAYA]

The letters, photos, and a transcript of the letters would all be public domain, since everything published in the US before 1923 is now in the public domain. The transcript does not get its own copyright, because it is not sufficiently creative to get protection.
However, your layout of a book about them would be copyrighted as soon as you make it, and any text you write about them in your manuscript would be copyrighted as well. You could register your copyright to guarantee your rights in that regard