Shang-keng Ma 
Modern Theory Of Critical Phenomena [PDF ebook] 

An important contributor to our current understanding of critical phenomena, Ma introduces the beginner–especially the graduate student with no previous knowledge of the subject-to fundamental theoretical concepts such as mean field theory, the scaling hypothesis, and the renormalization group. He then goes on to apply the renormalization group to selected problems, with emphasis on the underlying physics and the basic assumptions involved.