This is the 60th most frequent Czech word.
kterou
"Which" (feminine accusative singular form).
Here, 'kterou' functions as a relative pronoun introducing a subordinate clause, referring to the book being read (feminine, accusative).
Kniha, kterou čtu, je velmi zajímavá.
The book that I am reading is very interesting.
Here, 'kterou' is adapted into its proper case to emphasize the reference and its relevance to the discussion (feminine, descriptive clause).
To je otázka, o které jsme diskutovali.
That's the question we discussed.
In this sentence, 'kterou' remains effective in this context, emphasizing relation to a feminine noun, indicating the connection to the preceding clause.