Say you know next to nothing about semisimple Lie groups, or even any kind of groups for that matter, but that you are math-literate and willing to read, then this book could be what you are looking for. It comes in a welcoming yellow hardback edition and there’s an errata online. Let’s be clear. Why not give yourself a treat and go and read: Lie Groups, Lie Algebras and Representations: An elementary introduction by Brian Hall. What else have you got to do?