In the first problem set of chapter 1, problem 1-8(b) deals with angle preserving transformations. In the newest edition of the book the problem is stated If there is a basis x_1, x_2, ...., x_n and numbers a_1, a_2, ...., a_n such that Tx_i = a_i x_i, then the transformation T is angle preserving if and only if all |a_i| are equal. In the first edition (the one with the cool blue diagrams that you can actually see), the absolute value bars are omitted on the a_i. However, I'm pretty sure there are counterexamples to both formulations. If the basis happened to be orthogonal, then equal absolute values of the a_i would be enough, but if the basis were oblique, then you would need equality of the a_i. But the author doesn't mention orthogonality until a couple exercises later. Am I missing something? thanks.