This book takes a couple of old ideas about language that seem ludicrous and discredited, and shows that there is something in them. If you have read Deutscher's "Unfolding of Language", the first thing to know about this book is that it's much easier to understand - I read it all in one sitting, which I can't imagine doing with the earlier book because of the fairly hard going when discussing technicalities of grammar.
One issue seems rather dry and academic, but turns out to be anything but - names for colours and their development over time, starting with a book about Homer, by Gladstone (yes, the Victorian PM), which drew conclusions about colour perception by the Ancient Greeks from descriptions like "wine-dark sea". Similar discredited notions are ideas like speakers of languages with complex sets of tenses having a more highly-developed notion of time than those who use fewer tenses or none at all. Deutscher shows how the desire to get rid of silly nonsense has resulted in some equally silly nonsense, like the tenet that all languages are "equally complex" whether they belong to an 'advanced' Western civilisation or a 'primitive' aboriginal group. Far more acceptable of course than the notion that the 'primitive' language reflects racial inferiority, but still nonsense, because we have no way of measuring how complex a language is - we may as well say that all languages are equally green.
The other dodgy old notion is that your mother tongue affects the way you think. Deutscher shows that in a few ways, it actually does. Along the way you get entertaining coverage of things like gender conundrums, including the fact that Mark Twain's joke about female turnips applied to Old English just as much as modern German, and that like "she" for ships, this turnip gender lasted way past the death of Old English as a language - Deustscher quotes an example from a medicinal manual published in 1561.
It's all presented far more clearly than my hasty summary of the ideas can show you, and there's much incidental interest along the way, illustrating the fine line between thought-provoking but carefully stated suggestions and false statements (possibly based on a desire to 'prove' those suggestions) that lead generations of academics down the wrong path. Deutscher is good at seeing this process and as fair to the various participants as he can be. He's also good at getting you to see that aspects of foreign languages that seem absurd to English-speakers may actually indicate what's strange about English. His mother tongue is Hebrew, which has the same "irrational gender system" as most European languages, and he tells us that "If I knew more about (feminine) ornothology, I could tell by looking at each bird what biological sex she was. I would point at her and explain to the less initiated: 'You can tell she is a male because of that red spot on her chest and also because she is larger than the females.' And I would not feel there was anything remotely strange about that."
The biggest recommendations: the fact that I read the whole book again within a week, and Deutscher taking over from Steven Pinker as my favourite author of language-related books.