"You are speaking of his book" is not a good translation. " You are speaking / talking about his book." This is what we would naturally say in English.
parler de should also mean "to talk about", and not simply just "to talk of".
I would hardly ever say "speaking of" and almost always say "talking about". The only time I ever hear "speaking of" is when you are talking about something and then that topic is brought up again by something or someone else. For example, if you are talking about a certain person and then they walk in and you dramatically say "Speaking of Charlie!"
Blenda919226, Quite so! We often say ' Speaking of which' (or 'speaking of whom',-possibly less often) but we 'talk about' rather than 'speak of' books 99% of the time. This is not an impossible preposition but very, very unlikely to occur. I mean 'We were speaking of her oven', for example, is awkward and just odd! 'Talking about' just flows!
why has this been assessed as incorrect today and "her book" has irrationally been assessed as the correct answer?