"How to Draw Fantasy Art and RPG Maps" by Jared Blando, should be required reading for any Game Master, whether Fantasy or Science Fiction, or somewhere in between! In its 128 pages are more than thirty examples of how to put your dreams of a new gaming world to paper. It really is step-by-step cartography for gamers and fans!
Written by Jared Blando, who is a professional cartographer and concept artist, it bridges a gap that allows even beginners to take the vision of their worlds in their minds and put them to paper.
Each example builds upon the foundations of the last. You can use this book to begin, or build out, or complete, a real paper map of the world your players are adventuring in.
The main reason, I think, that you should do this, is to not only to give your world 'flesh' - but to make sure that you don't make the mistake of forgetting where your characters started, where they are now, and where you would like them to go at least.
I believe (and this has happened to me) that players might and probably will think about the places they've been, and expect those places to still be in the same locations, more or less. It might cause players to lose interest if they believe that, for instance, your capital city is in the west, and then suddenly find that you now say that same city is in the east.
A map of your world can serve another useful function - what happens if your players go a completely different direction than the one that you so methodically set up? Having a drawn map would make life a little easier if you know where more than one area to explore is! And, if your players decide to go north when you wanted them to go west, a map should show areas around the current players location so that you and they don't go too far astray.
So what if your Northlands are an expanse of frozen wastes? If the players know that's where they want to go, then with a printed map, you can provide details about this winter wonderland which may deter them from going there. As a 1st level character, I don't know about you, but I'd probably not want to go north if you know in advance that the Northlands in your world are filled with Frost Giants, Dragons, and other deep northern denizens!
In addition, the rich illustrations in this book could conceivably be used as your fantasy world map itself!
To wrap up then, I have found this book an invaluable resource for Fantasy and other role playing game map creation. If you have ever held the thought that you might like to actually see what your world looks like, this would be one of my first choices.