English grammar recognizes a distinct "determiner" function that is carried out by certain words that precede a noun. Only a limited set of words can be used as determiners. For example, the pronoun it cannot be a determiner, but the definite article the can. The word my is used as a determiner ("my book") but the word mine is used in other contexts ("That book is mine", "Mine has a blue cover").
The word who cannot be used as a "determiner", so *"who friend" or *"who American Author" are not valid noun phrases. "Who" would be used in a sentence without a following noun, such as "Who did you hang out with?" or "Who wrote a series of books about a 10-year-old girl name Anastasia Krupnik?"
The word what is used as a determiner, and in this function can be used regardless of whether the following noun is animate or inanimate. Which can also be used as a determiner with either animate or inanimate nouns; there is a previous question about when to use what and which.