One - The book was seriously flawed in plot or delivery. It was however not so bad as to not finish it.
Two - The book had lapses in plot or delivery, but still kept my interest.
Three - The book was well written and mildly engaged me in the plot, are in its imagery. It left me wanting more, in a negative sense, as if something was missing.
Four - The book was wonderfully written and transported me into its plot-world during the reading.
Five - The book was superbly written. It transported me into the plot-world and had me anticipating what was coming next even in the times I was not actively reading it. It left me wanting more in a positive sense. Bring on the sequel!