I have just finished the series and have absolutely loved the books. I wish there were more! wanted to have a poster one my wall of The Long Earth book artwork, minus the text, but I couldn't find anything. So I created my own, inspired by the cover. Below is a link for a 50cm x 70cm with 5mm bleed pdf of a poster that you can get printed at your local printer. It is pretty shoddy photoshop work but I think it will look good in a frame, as long as you don't inspect it from 5cm away!
I've made it a little darker to fit with the rest of my pictures, however I can do it in monochrome colours, lighter, darker etc. if anyone is desperate