Learn Spanish (Latin America): ¿Qué libro necesitas?

In Spanish, the verb itself tells you who is doing the action. The -as ending in necesitas specifically means "you need" (informal). Also, Spanish doesn't use helper words like "do" to make questions; you just use the verb as is.

I learned that cuál means "which". Why don't we say ¿Cuál libro necesitas?

In Spanish, when you put a question word directly in front of a noun (like libro), you almost always use qué. Even if you mean "which book," you still say qué libro. The word cuál is generally not used right before a noun.

Could I add the word tú (you) to this question?

Yes, but it isn't necessary. If you wanted to emphasize it, you could ask ¿Qué libro necesitas tú? or ¿Tú qué libro necesitas?. But since necesitas already means "you need," leaving tú out is the most common and natural way to say it.

Why is there an upside-down question mark (¿) at the beginning?

Spanish requires an inverted question mark at the start of a question. This is a helpful signal for the reader to know immediately that they should use a rising tone of voice, even before getting to the end of the sentence.