¿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.

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.

Ser is used for essential characteristics, classifications, and things seen as more permanent.
Here, ser is used because you are identifying a book as your favorite, which is treated as a characteristic, not a temporary state.

So:

• ¿Cuál es tu libro favorito…? = Which book is your favorite? (identity/characteristic)
  Using estar (¿Cuál está tu libro favorito?) would be incorrect in this sense.

In Spanish, most adjectives come after the noun, especially when they simply describe or classify the noun.

• libro favorito = normal, neutral order
• favorito libro would sound wrong in standard Spanish.

Some adjectives can go before the noun (e.g. buen libro, gran libro), but favorito almost always goes after: mi libro favorito, su deporte favorito, etc.

Adjectives in Spanish must agree in gender and number with the noun they modify.

• libro is masculine singular → libro favorito
  If it were plural or feminine, the adjective would change:

• mis libros favoritos (my favorite books)
• mi película favorita (my favorite film – película is feminine)

Tu is the informal singular possessive adjective: your (talking to one person you address as tú).

• tu libro = your book (informal, one person)
• su libro = his / her / your (formal) / their book
• tuyo is a possessive pronoun, not used before a noun:
  
  • Este libro es tuyo. = This book is yours.

So in tu libro favorito, you must use the possessive adjective tu, not tuyo.

You change tu to su, keeping everything else the same:

• ¿Cuál es su libro favorito en español?

Here su means your in the formal sense, referring to the person you treat with usted.
Context will tell if su means his, her, your (formal), or their.

Spanish usually does not use a definite article between a form of ser and a possessive + noun in this kind of structure.

So:

• ¿Cuál es tu libro favorito? (correct and natural)
• ¿Cuál es el tu libro favorito? (incorrect)

The possessive (tu) already makes the noun specific; you don’t need el as well.

Yes. En español means “in Spanish (language)”.
So the question is asking:

• Which is your favorite book that is written in Spanish? (or that you read in Spanish)

If you wanted to ask about a Spanish class textbook, you might say instead:

• ¿Cuál es tu libro de español favorito? (more like “your favorite Spanish textbook / Spanish course book”)

Yes, but the most common and natural position is at the end:

• ¿Cuál es tu libro favorito en español?

You could also say:

• ¿Cuál es tu libro favorito, en español? (with a slight pause, more like “…in Spanish, I mean”)

Placing en español in the middle (¿Cuál es en español tu libro favorito?) would sound unnatural in everyday speech.

They are similar in meaning but with a nuance:

• ¿Cuál es tu libro favorito en español?
  Focuses on identity: which single book holds the status of favorite.

• ¿Qué libro te gusta más en español?
  Literally: Which book do you like more in Spanish?
  This gets at preference by asking which you like most, but it doesn’t use the word favorito.

In practice, both can be used to ask about your top choice, but cuál es tu libro favorito sounds more standard for “What’s your favorite…?” questions.

Spanish uses both ¿ and ? to clearly mark where the question starts and ends.
This helps with:

• Intonation when reading aloud
• Long sentences, where the question part might begin later in the sentence

So ¿Cuál es tu libro favorito en español? must start with ¿ and end with ?; using only the final ? is considered incorrect in standard written Spanish.