Four artists share their techniques for mapmaking as well as samples of their work in The Art of Map Illustration. The book is full of beautiful illustrations by each of the artists, who employ a variety of media including pen, ink, watercolor and digital.
There are a number of map-making tips spread throughout the book, but it seems more time is spent describing how to illustrate map embellishments such as trees and buildings. Each of the artists share an almost whimsical style (as seen on the cover) with cartoonish illustrations and that probably accounts for the number of pages devoted to creating and placing those decorative details.
If the style suits you, the The Art of Map Illustration (Quarto Publishing Group, digital galley) is full of samples and would be a good book to reference for inspiration. Although the artists use a variety of media, the book feels a little repetitive because of the similar illustration styles.