A Question About Citing Sources

Didn't that title get you all excited about reading this post?

Okay, I'm still enjoying Susan Cheever's American Bloomsbury. Oh, that Ralph Waldo Emerson! What a guy! But I have a question about the endnotes. There aren't a great many, and they aren't marked with numbers in the text. So I have to guess what material might have a note citing its source. And none of the things I expect to have citations have citations.

For instance, when an author of a piece of nonfiction says that someone actually said something, doesn't that have to be cited? When you state that both the Thoreau boys were in love with the same woman and each proposed to her, doesn't that information have to have a note stating where you found out about it?

Or is that just the case when you're an undergraduate student?

I'm not questioning Cheever's scholarship because last year I read a memoir/account of a murder (Girls of Tender Age by Mary-Ann Tirone Smith) that did similar things. In fact, in that book the author gave accounts of conversations among police officers that took place a few decades back with no citations regarding how she came to know of them.

That book, too, was quite decent reading.

So my question is, has something happened regarding the rules for citing sources? Do particular types of books--maybe, popular history, say--not require the same rigorous documentation?