Could anyone explain it to me why it is not Denne er en ... since the gender og sang is -en ??
Im a dane.
In natural danish we would neither use denne or dette.
We would say "Det er en spansk bog" = This is a Spanish book.
I'm not 100% sure of it, but it's probably just a rule that only dette is used, like how only det er (it is) is used and not den er.
This was explained to me in another topic, but I believe that any time the subject is not yet known, you use dette and det.