Beautiful map: the colours are vibrant and the biomes are very well rendered, pleasing to the eye and meaningful.
You've taken great care with the details, both on the map and on the border (which is incredibly precise).
I admire your initiative too: how many fantasy books don't have a map (sacrilege!) or have an ugly map that isn't even credible (but what is the river police doing?)
You were right to offer your help, it's all to the benefit of the author and the readers!