Well, to CYA, I would get permission. You can talk to a lawyer about
it, and that would probably be best. For initial contact though, if
you're taping the interviews, get the interviewee to identify
themselves and state that they have agree to talk to you and that they
know you will be using the information from the interview in the book
(name it). If you hand write, get the same information at the front or
end of your notes and have them sign it.
Newspaper archives are public record. However, they are copyrighted so
you need to cite them same as old books. Diaries should be cited and
the current owner should sign that they know what you're working on and
that they gave you permission to read the diary.
I think that covers what I know from a journalistic perspective.
Lynn