I have to say that this is the first image I've constructed for a black and white book print so I've mostly relied on the (more comprehensive) advice elsewhere on here to sort out the technical details. A few things that I've picked up as I've gone along though in the process have been interesting.

1. Use the Gimp presets to get a 300dpi A5 landscape document.
2. Work at 100% when drawing the details - otherwise the ink tool can often look pixelated when you go back to 10% even though it shouldn't
3. Uncheck View Dot for Dot (under the view menu) whenever you need to step back and see how it looks at the correct scale.
4. Work in greyscale and then change to indexed black and white at the very end.

To that end, here is a version with changes that Redrobes suggested to the river banks and coast. I have also behaved myself and put this into black and white and made sure its the right pixel dimensions. I also replaced the compass that got lost when I output the map without the labels.

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Now that I've put it out in black and white I can see a few things that need to be changed - the compass could do with a bit of darkening for one.

DETOUR - Did you know that in Gimp 2.6 if the main window has focus and you press tab once, the other Gimp windows get folded into the main window in the windows bar at the bottom so that there is only one entry for Gimp? Just noticed it today - nice little change in my book.