Sketch Your World has been reprinted and is on shelves and in online warehouses again now. I dropped in on the recently renovated Tate Britain – through the main entrance past groups of art students sitting on the steps, which suddenly transported me back to my student days – to visit its bookshop where copies are on sale. (It has a different cover if you are in the US or Asia.)
There's more information about the book at its Facebook page. Let me know if you have problems getting hold of a copy.