Suppose you are looking at a red screen. The usual view is that this produces the sensation of redness, which you interpret as perception of the colour of screen, a fact that we can then use in our interaction with the world around us. Humphrey modifies this. He argues that the perception could take place without the sensation - that the sensation of redness is an added extra. He put forward some convincing arguments, in particular the phenomenon of blindsight, where people can interact with objects without believing that they can see them. His view isn't just the rather doubtful one of epiphenomenalism though, rather he thinks that this sensation can affect our emotions, as well as giving a way of replaying experiences in our mind.
Of course this a book like this isn't going to settle all of the arguments about consciousness, but I feel that Humphrey's idea is important as it gives some structure to the questions involved. It's a short book - no need to wade through pages of obscure philosophy - and I would recommend it to anyone with an interest in the nature of consciousness.