I'd suggest looking at the maps people have posted, and see what you like. There's all different kinds of maps styles.

But, you might want to hold off. If you intend to try and get this book published, the publisher would pay for the map to be made, and can pay the artists a fair price.

For example, for a black and white map, I tend to get between $800-$1000, and still retain the copyright. Can you afford around $500 (low end publishing rate)?

And you should wait till you are done. Your book can change, and if the map is done before you are, asking an artists to make changes after they are done is asking them to work for free.

Even when I do maps professionally, details can change, as the author and editors remember things differently.