Questions & Answers about Se on kirja, jota luen nyt.
It can feel like either, depending on context. In this sentence, se refers to some specific thing and can be understood as it or that in English.
Finnish se is often used for things that are already known from the situation or have just been mentioned. English usually has to choose more clearly between it and that, but Finnish is often less strict here.
Finnish does not have articles. So kirja can mean a book or the book depending on context.
The sentence becomes specific because of the relative clause jota luen nyt, which tells you which book is meant.
Because kirja is a predicate noun after on.
In a sentence like Se on kirja, Finnish normally uses the nominative singular for the noun that identifies or classifies something. So kirja is the expected form here.
jota is a relative pronoun. It links kirja to the clause luen nyt.
In English, this is the kind of word that often corresponds to that or which, as in the book that I am reading now.