Learn Spanish (Latin America): ¿Cuáles son tus libros?

In Spanish, the question word cuál (which) has to match the number of the things you are asking about. Since you are asking about multiple books (libros), you must use the plural form cuáles (which ones).

Why do we use son here instead of están?

We use son (from the verb ser) because we are identifying things. The verb ser is used for identity, such as pointing out which specific books belong to you. You would use estar for location, like if you asked where the books are.

Since I am asking one person, why is it tus instead of tu?

In Spanish, words like tu (your) must match the things being owned, not the owner. Since one person owns multiple books (libros), you have to add an s to make it tus libros.

Could I say ¿Qué son tus libros? instead?

No, that would sound strange! In Spanish, you use cuál or cuáles when you are asking someone to make a choice from a group of options (like pointing out your books from a pile). If you used qué, you would literally be asking for a definition, like asking what the concept of a book is.