"Jeg leser boken selv om du hater den."
Translation:I am reading the book even if you hate it.
It's a little ambiguous whether the 2nd person hates the book, or the act of it being read. The 2nd seems more silly to me, so I'll go with that.
It's not ambiguous in the Norwegian sentence. If the 2nd person had hated the act of it being read it would've been "Jeg leser boken selv om du hater det".
The reasoning is simple, 'bok' is a masculine noun, so you'd have to use the masculine/feminine demonstrative, 'den', to refer to it. 'det', however, refers to either an neuter, or something more abstract, which in this case is the act of reading. Another way to phrase that would be "Jeg leser boken selv om du hater at jeg leser (den).", so 'det' would be the replacing 'at jeg leser (den)'.
'jeg leser (den)' has already appeared in the sentence, so 'det' is used to avoid saying it twice.
I thought bok was feminine, so I checked a dictionary, and apparently it can be either masculine or feminine. (?)
It's actually feminine, but all feminine words may be inflected as if they were masculine. I've corrected my statement; 'den' can refer to a masculine and a feminine noun.