i’m currently writing a comparative book review in which an author uses direct quotations from a private letter between two military generals but it’s not cited. i tried looking for it in their bibliography because they give the date of the correspondence when discussing it but i couldn’t find it.
Yeah, generally true if the author just uses it. Sometimes more popular history won't have footnotes but will ( hopefully) have the relevant sources in the bibliography for sections of the book. But it is very specifically true if the author actually quotes the source. Even if the letter is in the hands of a collector who wishes to remain anonymous, there should be a citation, with the location of it being, like, "private collection", or, if it is owned by the author, "author's collection". As a good scholar, the author is supposed to say where the source is located.
There are many different citation styles, but this actually sounds like an error. Private letters are generally drawn from an archive. There should have been a footnote or endnote explaining which archive was used. Failure to do this means that anyone who wants to verify the contents of the letter is going to have to hunt down every public archive that contains such correspondence.