"Who", like "you", can refer to either singular or plural. If you know that two people are speaking, then you would ask, "Who are speaking?" Otherwise, if it's not clear, you would use the singular, "Who is speaking?"
It's not that the book is wrong, rather if you don't know how many are speaking, you would use the singular by default. Once you know there are multiple people, you change the verb to match:
"Who is speaking?"
"Both John and David are speaking"
"Oh. Who are they?"
"They are the ones scheduled to speak."
You should use the singular when querying a group, but again change the verb if referring to multiple people:
"Who wants ice cream?"
(three children answer "yes")
"OK, what flavor to you want?"
It's the same way if the two people are talking to each other:
"Who is in the office with the boss?"
"Jim and Mary."
"Ah. What are they talking (to each other) about?"
Lastly, you should use the plural if the subject is obviously plural from the context of your question.
Who are the executive members of the committee?
Who are the students who will receive the award?
Who are those people waiting by the elevator?
and so on.