Unputdownable. A book so well written and entertaining it is difficult to put down.
Forget rich media and instant messenging, there is nothing like a good book to keep us up late on a school night, whether that's a printed or Kindle one.
A book that is so involving you don't want to stop reading is "unputdownable" or "a page-turner".
It might be great literature - like the brilliance of The Great Gatsby where you feel like clapping at every clever sentence - or the incredible The Heart of Darkness, which keeps you turning the pages, getting more and more depressed but compelled to continue.
But it might equally be an airport book or chick lit. The only essential requirement is that it tells a good story. What unputdownable books have you read lately?