Learn French: Il y a mille livres dans l'école. - There are a thousand books in the school.

It means both! Il y a never changes form. Even though we are talking about a thousand books, which is plural, we still use il y a for 'there are'.

Why is it mille livres and not mille des livres?

When you use a specific number like mille, it acts as the quantity and replaces plural words like des (some). You just put the number directly in front of the noun.

Does mille ever get an 's' at the end?

No, mille is a special number because it is invariable. This means it never changes and never takes an 's' at the end, even if you are saying deux mille (two thousand).