An Introduction to the Theory of Groups by Joseph J. Rotman
Anyone who has studied abstract algebra and linear algebra as an undergraduate can understand this book. The first six chapters provide material for a first course, while the rest of the book covers more advanced topics. This revised edition retains the clarity of presentation that was the hallmark of the previous editions.
From the reviews:
Rotman has given us a very readable and valuable text, and has shown us many beautiful vistas along his chosen route. --MATHEMATICAL REVIEWS