Questions & Answers about Boken har hundre sider.
Because boken is the definite singular form: the book.
In Norwegian, definiteness is often added as a suffix to the noun:
- en bok = a book
- boken = the book
So in Boken har hundre sider, the speaker is talking about the book, not just a book in general.
That is a normal feature of Norwegian. Instead of usually having a separate word like English the, Norwegian often attaches definiteness to the noun itself.
For example:
- en bil = a car
- bilen = the car
- ei bok / en bok = a book
- boka / boken = the book
So -en in boken works like English the.