"It is a book."
Translation:Det är en bok.
In this sentence, det does not refer to the book. It's there because Swedish sentences must have a subject. Compare it to it rains in English, where it doesn't really mean anything, but the sentence structure of English needs it.
When Swedish does it like this, the subject is always det. Some examples:
Det regnar = it's raining
Det är fint väder ute = The weather is lovely outside
Det är fjorton bilar på gatan = There are fourteen cars on the street