The conjugation of "escrever" for "nós" is the same for present and simple past tenses :/ However, if this is a beginners lesson, they probably meant it to be the simple present...
Gaspard is right. (It could be We wrote a book as well, but that is not for begginers)
Plus: "nós" is the "us" complement when it comes after a preposition:
- Nós nos olhamos (we looked at ourselves)
- Eles olharam para nós (they looked at us)