H. F. Jones, Groups, Representations and Physics, 2nd ed., IOP Publishing (1998). A fairly easy going introduction. H. Georgi, Lie Algebras in Particle Physics, Perseus Books (1999). Describes the basics of Lie algebras for classical groups. J. Fuchs and C. Schweigert, Symmetries, Lie Algebras and Representations, 2nd ed., CUP (2003). This is more comprehensive and more mathematically sophistic...