Learn Hardcore Spanish (Spain): ¿Cuál es tu libro favorito en español?

In this question, the speaker is asking you to choose one item from a known type of thing: your favorite book.
In Spanish, cuál (or cuáles in plural) is often used when:

You are choosing from a set of possibilities, and
The noun is already clear or mentioned in the sentence.

¿Cuál es tu libro favorito…? literally feels like “Which is your favorite book…?”

You could say ¿Qué libro es tu favorito?, but it sounds a bit more marked; the natural, standard way here is with cuál + es + noun phrase.

Why is there an accent on cuál?

The accent on cuál shows that it is being used as an interrogative word (in a question) or exclamative (in an exclamation).

¿Cuál es tu libro favorito…? = question → accent.
No sé cuál es tu libro favorito. = indirect question → still accent.

Without an accent (cual), it is usually a relative pronoun in more formal or literary contexts, and it is much less common in everyday speech.

Why is it es (from ser) and not está (from estar)?