In the Genki 1 textbook, the question prompt
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,
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？
who is reading.
what is Takada-san doing. In other way you can ask
本を読んでいる人ですかis wrong. And answer will be
高田さんです. Sorry, I can't explain it correctly in grammar terms.