ei bok · book
Feminine (ei)
Indefinite singular
ei bok
Definite singular
boka
Indefinite plural
bøker
Definite plural
bøkene
Feminine nouns (ei) can always be written with en as well: ei bok, boka or en bok, boken. Both are correct Bokmål.
How to guess the gender Nouns: gender and articles