This book is an excellent second book after the one from J Rotman (An introduction to the theory of groups).
Self studying with the 2 books demands a lot of work but can be achieved by someone motivated. When a concept is not understood with one of the books, the explanation read in the second really help.

This book is not for a beginner, but is perfect for someone with a real mathematical background. The concepts are organised as can be the mathematical courses in universities, concise, succint and demanding.