This is the second book in the series of books entitled Theoretical Minimum based on the lectures given by Leonard Susskind at Stanford University, aiming at introducing modern physics to a general audience. If you are familiar only with vector spaces, basis, linear maps, matrix representations of linear maps with respect to basis, then you may challenge the book after reading the first book. That's the merit of the book. With a minimum amount of mathematical and physical background, it helps readers to understand the essence of quantum mechanics.<br><br>I think that one of the virtues of a well-written book, especially that of a book for a general audience, is its reasonability in unfolding its stories. Readers should be able to follow the book thinking "so... ok... so... ok". Overall, the book has the virtue. But at several parts of the book, it falls short of my expectation.<br><br>1. At the first try, I stopped at Lecture 7 concerning entanglement. I did so because the explanation was confusing and the confusing part seemed a bit long. On the second try, I skipped the part and back to the part after reading some later pages. I realized that the responsibility was not mine, but of the authors. The explanations of the Lecture 7 were not clear, many parts were redundant, and moreover, the order of explanations must have been reversed. If you are confused in reading Lecture 7, I would like to recommend to skip it because you would have no difficulty in finishing the book even if you skip the whole part.<br><br>2. There are some typos and they tend to make readers to misunderstand. For example, at page 165 of Lecture 6, it says,<br><br>A tensor product is a vector space for studying composite systems. A product state is a state-vector. It's one of the many state-vectors that inhabit a product space. As we will see, most of the state-vectors in the product space are not product states.<br><br>This should be replaced with<br><br>A tensor product space is a vector space for studying composite systems. A product state is a state-vector in the tensor product space. It's one of the many state-vectors that inhabit the tensor product space. As we will see, most of the state-vectors in the tensor product space are not product states.<br><br>And in later chapters, the usage of two words probability and probability density is incorrect.<br><br>3. The authors seem to assume a few things that are not proven in the text. For example, they assume that the trace of a linear map is independent of a choice of a basis. Although this is not shown in the book, this is a mathematical fact. And they seem to assume that any hermitian operator represents an observable. I think it cannot be true.<br><br>Now, I have finished the book. If someone asks me what quantum mechanics is, then I will tell him first about classical mechanics. Next, I will talk about state-vectors, Schrodinger equation, how to get information about position, momentum, energy from state-vectors. And about observables, expectation value, uncertainty principle, entanglement and quantization of energy in bounded systems. They are all totally different from the corresponding theories of classical physics in their nature.<br><br>The book concludes with a discussion of harmonic oscillators. Without any reference to quantum field theory, the authors say that it is the gate to quantum field theory. They seem to assume that we heard of quantum field theory and its greatness. To me, the final chapter was unsatisfactory. I chose the book, not because I wanted a prerequisite of quantum field theory, but because I wanted a book telling me what quantum mechanics is. I think it would better conclude the book with a section that summarizes what has been dealt with and says what quantum mechanics really is in several paragraphs.