General question, do you think it would be easy to read Harry Potter in Norwegian?
As Linn already mentions it's certainly doable! Of course it helps if you have already read the books before so you know the general storyline and can easily fill in any gaps in Norwegian knowledge you may encounter. Reading them as ebooks with a built-in dictionary is also very handy as you may encounter many new words. I'm almost finished with the last book myself and so far have a list of over 500 words which aren't included in the Norwegian Duolingo course (although some of them are the Norwegian versions of names as they tend to be different as bandi.cute already stated).
Another thing which took me a bit by surprise in the beginning (although it's perfectly logical) is that many verbs are in the past tense: "«Potter! Wiltersen! Følg med!» Professor McSnurps irriterte røst smalt som en pisk gjennom torsdagens transfigurasjonstime, og Harry og Ronny skvatt og så opp."
I actually just bought the ebook version of the first book. It's definitely a challenge, but really fun! It's also cool to see what names are different (though a bit confusing at first).
As a native German speaker with a lot of English knowledge, I didn't find it too difficult after having completed the Duo tree. But I'd say German helped more than English.
Just bought the first book a few days ago and I can say it is quite challenging! But it's definitely doable if you have time and patience.
So, "trappa" is female and "trappen" the male word for the stairs, is that correct?
Well "trapp" is female (ei trapp), but in Norwegian you may use both the female (trappa) or male (trappen) definitives for female words.