"It is a book about horses."
Translation:Det er en bog om heste.
No "den" would not be correct here. Here "it is" a general statement in the sense that it would make no sense to swap "it is" with "the book is". ("The book is a book about horses" makes no sense)
If the conversation went like "what is the book about?" "It is about horses" you could use "den" because you are directly referring to the book. (Here you can replace "it" with "the book", as in " the book is about horses").
But the sentence "The book is a book about horses" does make sense to me? It's awkwardly phrased, nobody would say it like that, but it does make sense. I guess this is just a rule I'm gonna have to learn.
that's weird, as I am told that even when pointing to something one should use "det" or "den" depending on the thing, e.g. if I point to a previously unmentioned blackboard I should say "den er sort".
Apparently det can be used very liberally in Danish, including with a plural object (det er mine katte). So in this case, both det and den would be acceptable, but det is kind of universal.