It's a very important book, written by the father of group theory application in physics (with Wigner and Pauli), and one of the best mathematician of 20th century, Hermann Weyl. Everyone who wants study a deeper view of quantum mechanics, in his intrinsic mathematical formulation, should read this work. After a firt brief introduction to quantum theory, he passes to explain the theory of rapresentation of groups, and its physical application, like the rotation group, or Lorentz group, and finally the theory of simmetry. It's a fundamental book for a good understandig of the importance of simmetries in modern physics. Without any doubt one the the most important work in quantum mechanics.