It's an incredible book, not really a commentary. Machen had written most of the articles in C. Today from 1931-33.
The very first chapter is worth the entire book on Gal. 1:1. The first word of the epistle: "No." He writes:
"It is a word that is now regarded as highly objectionable, a word that Paul if he had been what modern man would have desired him to be, never would have used.... That word 'not,' we are today constantly being told, ought to be put out of the Christians vocabulary. Our preaching, we are told, ought to be positive and not negative; we ought to present the truth, but ought not to attack error; we ought to avoid controversy and always seek peace. With regard to such a program, it may be said at least that if we hold to it we might just as well close up our New Testaments; for the New Testament is a controversial book almost from beginning to end."
AMEN!
Buy it! It took me over 10 years to find it used, and then it was reprinted! Enjoy.