Sadly, while I have seen this issue in many self-pub books written and “edited” by subliterates, this is becoming much more common in books from tradpub houses, as well.
ONLY, and I do mean ONLY, in the case where a subjective pronoun is used in a predicate nominative renaming of the subject is using a subjective pronoun in an otherwise objective position proper. The definitive example is: “I am he,” from which it follows that “You are she,” etc., are proper.
unfortunately, all too many subliterate “writers” (and yes, speakers) insist on thinking that ALL (or most–some are amazingly inconsistent) personal pronouns in objective case position) within a predicate are subjective case, based on this one exception (where the initial subject is used, as previously noted, to simply rename the subject they simply misuse a subjective case pronoun, viz.,
“I am better than he.” [WRONG]
“I am faster than he.” [WRONG]
For some reason–possibly because they mistakenly think it sounds “classy” or some such–many subliterates cling to such uses. Of course, rarely are the examples as plain, as stark, as the ones I have given above. Most often, the predicate is complex, poorly-constructed, and amphibolous, as well, obscuring the objective case.
I find this annoying. OK, in dialog, as long as the writer is trying to establish the speaking character as either barely literate or illiterate, OK. Barely. But when it occurs in descriptive narrative, no matter how otherwise interesting the content–fiction, non-fiction–I am sorely tempted to trash the text and move on to* something else. Time is just too precious to spend it translating badly-written text.
*Another niggle, one that is a sure sign of extremely poor literacy in a writer: using “on to” or “in to” in place of the prepositions “onto” or “into,” as well as the converse. Ditto for many others, as well as compound words misused when the separate words are called for (“everyday” for “every day” for but one example–the meanings are very different, at least to literate people.), and, again, vice versa (using separate words–“any thing” for “anything,” for example).
This is not the end. I have a million** of these.
**This may be hyperbole. And no, not “maybe hyperbole.”
BTW, the reasoning that “he” in “I am he” is pretty much retrospective, and based largely on literary references from the 17th Century, etc. For example, in John 18:6, the KJV (which has been a stronger influence for the standardization of English grammar than perhaps any other text) has Jesus answering, “I am he,” though this is a weak reed to lean on because the Greek text the translators were working from there has (transliterated), “ego eimi,” which is simply “I am.”
So, my opinion differs from allowable “proper” or formal usage from the nominative substitution excuse *heh* to personally prefer “I am him” whenever one mistakenly fails to say either just “I am,” or “it’s me,” or some such clear statement.