This is the 555th most frequent Japanese word.
どちら
"Which" or "where" (polite form).
どちら is used here as an interrogative pronoun asking for a choice between options (books).
どちらの本が好きですか？
Which book do you like?
どちら refers to the origin or place someone comes from, used here interrogatively.
どちらから来ましたか？
Where are you from?
どちらも indicates inclusion of both choices, emphasizing inclusivity.