The Classical Groups
The Classical Groups: Their Invariants and Representations is a mathematics book by Hermann Weyl published in 1939. The book describes classical invariant theory in terms of representation theory. It is largely responsible for the revival of interest in invariant theory, which had been almost killed off by David Hilbert's solution of its main problems in the 1890s.
The second edition was published in 1946. Weyl gave an informal talk about the topic of his book in 1939.