Translation:The woman is reading the book because it is interesting.
Why "den er interessant" and not "det er interessant"? Shouldn't "it" be translated to "det"?
'it' should translate to 'det' or 'den' depending on what it means in the Norwegian sentence.
In this case 'den' replaces 'bok' in the dependent clause. 'den' replaces a masculine or feminine noun, while 'det' replaces neuter.
"Kvinnen leser boken fordi boken er interessant" -> "Kvinnen leser boken fordi den er interessant"
You wouldn't say the first sentence, but that's what the second sentence would mean if you were to replace 'den' with the noun it replaces.