Well, forget the tourist pamphlets, that collect old folktales from the 19th century. Those were created in an attempt to sort out folk stories from the many traditions of Icelandic settlers. Truth is, there are no trolls, not as a non-human, humanoid species.
There is, however, a human ability to centre landscapes in human form. It is this centring, this inseparability from place, that you will find in Iceland, if you wander there outside of books. The secret of trolls is the secret of recognition, because they are the same thing. Many Icelanders today look to New York or London for their mirrors. Not all. You don’t have to, either. A troll is where you find it. You are where you find yourself. Now, recognizing yourself when you see it, ah, now that’s a trick.