This book takes the reader from the end of introductory Lie group theory to the threshold of infinite-dimensional group representations. Merging algebra and analysis throughout, the author uses Lie-theoretic methods to develop a beautiful theory having wide applications in mathematics and physics. The book initially shares insights that make use of actual matrices; it later relies on such structural features as properties of root systems.
Indispensable for anyone wishing to understand Lie Groups, algebras and root systems. I wish some examples were more developed and a bit more explicit at times, but Knapp comes with an excellent index and list of notation that makes navigation a breeze.