You own the copyright to the original creative content of your work. So if you drew the outline of the shapes of the land then thats the bit you own copyright to. When you commission and artist then they would own the copyright to all the bits they draw on your maps land mass. You can assign copyright rights to someone so when getting a commission done you can ask for the copyright to the final map. However this generally means that if the artist uses premade stamps and icons and other bits they have previously drawn or reused then now they cant use them any more. Consequently, some artists do not sell copyright and others charge a lot more for it because maybe every single pen stroke will now have to be unique for your map. Also without some copyright ownership or license, the artist cant even show the map as part of their portfolio.
Original creative content means that it basically has to have not existed in the past. A generic island of rainforest is not a copyrightable thing. The specific shape of your island might be but the fact that its a rainforest is not.
You do get copyright by default in most countries in the world - all that I can think of. You do not need to register it but at the end of the day you only really have rights to whatever you can defend in court. So you would have to be prepared to pay legal fees to defend it and also must prove that its your own and original. If you can find a similar looking island from somewhere else which was created prior to yours then maybe all of a sudden, your island shape is now owned by that previously discovered map maker or that its now public domain. During copyright court cases, people look for "prior art".
Public domain is where it is basically not copyright any more. This could be because the time limit has run out - which is about 75 years. So most stuff prior to 1920's is copy right free. Also, you can dismiss your own rights and declare something public domain when you dont want to care about it. The issues with declaration is that now that you have no claim to it someone less ethical else might say they claim it and put up the money to defend it. As a consequence many people go for the inbetween state of keeping the copyright but licensing the work under a creative commons license.
If you own copyright on a work or bit of art you are free to license it in any way you want to. If you say "All rights reserved" then it means that nobody else has the right to copy it. If you put a creative commons license on it then its stating that your giving an automatic permission to make some form of copying without the copier needing to individually ask for your permission. There are a range of options with a creative commons and you will see tags like CC-BY-SA etc which means you have to attribute it and all further copies need to be licensed in the same way. Its a little complicated but its a great way of allowing people to use your work without having to get into any licensing discussions.
If you want to write a novel in a land mapped by someone else then you are free to do so. Copyright covers the original artistic work. A new artistic work which is substantially different is not covered by it. However if you were to make a second map of the first one which included substantial aspects of the first which was copyright then you are creating a derivative work based on the original which is then considered a copy. The aspects of your new map copied from the original of which were copyright is an infrigement.
My personal feelings about it is that if your wanting to create a book then start from scratch and do all the work yourself. And if you commission a map for it then pay the extra money and buy the copyright for it and dont use bits of any other work in the book.
Although there are always going to be people who compare books like Lord of the Rings with Harry Potter, they are not the same and share no substantial aspects of each other. The main wizard having a beard is not enough. Its difficult to be truely original where there are not even vague aspects which are similar but it is not hard to be substantively different to avoid the possibility of copyright claims.
Hope that helps.