What do you think about that ?
Where do you usually go in the evenings?
Which of these shirts do you like most?
In all of these sentences the question is about the object while the subject is fixed - you. So you could try to construct similar questions with who:
Who(m) do you usually greet first when you arrive home?
Who(m) do you ask about your English questions?
Who(m) do you trust?
Who(m) do you fear?
Since above the pronoun seems to substitute the object I guess whom should be used. However I think that who could be also accepted, take a look at this recent thread for example: http://www.usingenglish.com/forum/as...ho-x-whom.html
So I conclude maybe the mentioned book was avoiding such somewhat more advanced grammar topic at the moment for didatic reasons.
P.S.: Not a native speaker