This is the 722nd most frequent Estonian word.
kellest
"Of whom" or "about whom."
In this question, 'kellest' is used to inquire about the subject of today's discussion.
Kas tead, kellest meil täna räägitakse?
Do you know about whom we are talking today?
The word 'kellestki' here indicates an indeterminate person who is being discussed in the book.
See on raamat, milles räägitakse kellestki kuulsast.
This is a book that talks about someone famous.
Here, 'kellest' asks for clarification regarding the subject or character discussed in the context of the story.