Et is the definite article for neuter nouns (et barn). Bok is masculine/feminine noun (ei/en Bok). Don't fall into the trap of trying to find a logic with the gender of nouns, you just have to memorize which belong to which genders.
It must to have a trick .. For example in spanish most of female words finished in "a" like "playa" thus we use the female article "la" instead the masculine articule "el" ...we have just some exceptions ..