This is an academic book written primarily for language experts and is hard going for people not up to date with linguistics. I think it is probably as clear as it could be - Pinker always keeps the general reader in mind.
The book tries to explain how we learn the very fine distinctions sometimes made by the syntax of verbs; I think he is saying it is bound up with the fine distinctions implicit in the meaning of the verbs.
This is a very different book from the ones written for the general reader; although Pinker seems to be saying that there are grand simplicities at the heart of his argument, it appears to the uninitiated to be complex and counter-intuitive at times.
My three stars reflect mainly my lack of expertise in the field - I don't really know how good it is!