* "We can borrow five books?! Awesome!" (This is simple and enthusiastic)
* "Five books?! We're going to be reading all week!" (This highlights the amount of reading you'll get to do)
* "Oh, boy! Five books – what a treat!" (This is more playful and excited)
* "Five books?! This is going to be amazing!" (This emphasizes the positive feeling)
The best exclamation sentence for you will depend on your tone and what you want to emphasize.