"He is not writing a book."
Translation:Han skriver ikke en bok.
"et" is neuter, "en" is masculine, "ei" is female. This is the grammatical gender and doesn't necessarily have anything to do with the gender of a person. For example, it's correct to say "en jente" (but "ei jente" works as well), even though "jente" is female.
This concept of grammatical gender might be a bit harder to grasp for native english speakers, as in english you only have one article: The. It's just something you have to try and memorize. But don't worry, natives will understand you even though you may use the wrong gender every now and then.
Actually, "bok" is a feminine noun, but in bokmål every feminine noun can be written as a masculine noun.
As to WHY "bok" is feminine/masculine and not neuter: I don't think even linguists know the reason for grammatical genders (or if they do, I haven't read about it). They don't follow any real logic, and you just have to memorize it.
In german, for example, "Buch" is a neuter noun, in french it's masculine (le livre). As I've said: There's no logic to it.