Naive Lie Theory

Description

In addition, someone recomended Stillwell's Naive Lie Theory as "a great first introduction"; it is an undergraduate text.

Recommended here as a good introduction to Lie theory:

I would suggest you start with chapter 4 of An Introduction to Manifolds by Tu, Then study Lie Groups, Lie Algebras, and Representations: An Elementary Introduction by Hall and finally study Differential Geometry, Lie Groups, and Symmetric Spaces by Helgason.