In the Genki 1 textbook, the question prompt

> 本を読んでいる人 _は_ 誰ですか？

appears alongside an illustration with a bunch of people doing different
activities. The textbook user is then supposed to pick out a specific person
in the image with は (高田さんは本を読んでいます).

I've been studying Japanese long enough to understand that in contrast,

> 本を読んでいる人 **が** 誰ですか？

sounds wrong, but when I take a second to think about it, I can't actually
figure out why it's wrong based on the meaning of が (it seems like it could
work as exhaustive-listing が if I didn't know better).

Thinking of は versus が as a difference between introducing new information （が）
and drawing upon old information（は）I was thinking maybe the nuance here is
that, in the textbook exercise all of the people in the picture are
established as pre-registered/old information (though I don't know _why_ they
would be considered old), and thus は is simply used to bring it into the realm
of the conversation again.

That guess aside, does anyone have insight on why は instead of が？
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2020-11-14T04:21:24.980
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question
syntax particle-は particle-が set-phrases は-and-が
は versus が in this sentence
I can't explain it correctly in grammar terms, but here is simple explanation
why you can't use 本を読んでいる人 **が** 誰ですか？

Case 1. You see man reading book and want to know who is it (is it Takada-
san?), so you asking 本を読んでる人は高田さんですか. And answer will be はい、高田さんです。 Yes, he is
mister Takada.

Case 2. You see two men. One of them reading book, another one drinking juice.
And you want to know is mister Takada a man who reading book? So you asking
本を読んでる人が高田さんですか. And answer will be はい、本を読んでいる人です。Yes, (mister Takada) a man
who reading book.

Okay, return to our question. 本を読んでいる人が誰ですか. Answer will be はい、本を読んでいる人です.
This is illogical question: Is who reading book? → Yes reading.
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answer