I just checked the latest edition of The Last Queen of England and 'Of whom' was cut from that sentence prior to the book being re-published with Thomas & Mercer. Are you reading an early copy?
Anyway, as you've raised the WHO/WHOM thing, following the who/he, whom/him rule WHOM is correct in the above sentence - even if it's not in the book anymore. :o)
When you switch the sentence 'WHOM I'm sure you've all heard of.' you get 'You've all heard of HIM.'
So as it's HIM, WHOM is used.
A commonly used phrase people often get wrong is 'WHOM shall I say is calling?'
HE is calling, so it should be 'WHO shall I say is calling?'