This book is about the language of thought - how the mind frames concepts about the world internally. Pinker uses both psychology (mainly evolutionary) and linguistics to pick apart what the language of thought is comprised of. This approach is plausible, to the extent it works, but limited given our current state of knowledge. Still, there's plenty of Pinker's usual interesting psychological facts in here.
I found the first part of the book pretty hard going. It's fairly dense and dry, and the bulk of his thesis gets set out here. The second part is lighter, and significantly more readable, and has him applying the thesis to particular things - metaphor, swearing and the use of implication in speech.
Overall, a good read.