This is a wonderful novel about twin worlds that share an atmosphere and what happens when the inhabitants of one world have to abandon it for the other via hot air balloon!! It contains suprising charactarizations and a very cool ecology all wrapped up in a fairly believable page-turning plot. What more could you ask for from SF? This is the first book i've read by Shaw and I now consider him to be one of the better SF/Fantasy authors.His sentence by sentence prose is tight and scans easy. He knows how to describe a scene or scenes well without glutting the reader with oceans of words. He does fairly well with characters and although the protagonist is a bit of a Conan/type, we like him and find him fairly interesting.I found myself remembering scenes days after I put the book down, and that's pretty damn rare. Check it out, if you can find it.