No, "brev" is a neuter noun, so you'll have to use the neuter declension:
a letter - the letter - letters - the letters
et brev - brevet - brev - brevene
Try to always remember the gender of a noun as you learn it.
I didn't realize that nouns in Norwegian have genders. Is it masculine/feminine and then neuter? How do you distinguish one from the other? Do all neuters take the "et" article and everything else uses "en"?
Norwegian has three genders, but feminine nouns can be treated as masculine nouns, so you can use two if you want to. I'd recommend you to just learn the feminine as well. These can only be distinguished by memorizing them, but there might be some patterns. All neuter nouns takes the 'et', masculine 'en' and feminine 'ei'(or 'en' if treated as a masculine noun).