You are never going to believe this: I was busy thinking up suggestions for "unputdownable" like:
"eine fesselnde Lektüre, die man schwer aus der Hand legen kann",
when I decided "just for fun" to type in Denglisch:
"nichtausderhandlegbar": Amazon got there before me: http://www.amazon.de/review/R2OY9TUMG2CKJ6
This is not to say that I'd recommend this - it could, of course, be an automatic translation, or a joker with a made-up word. Language lives, however, and sometimes words get in through the back door via Google ...