In the process he gives a very clear delineation of the essence of biblical Christianity. For example on the topic of sound doctrine he says,
"Christianity is based, then, upon an account of something that happened, and the Christian worker is primarily a witness. But if so, it is rather important that the Christian worker should tell the truth. When a man takes his seat upon the witness stand, it makes little difference what the cut of his coat is, or whether his sentences are nicely turned. The important thing is that he tell the truth, the whole truth, and nothing but the truth. If we are to be truly Christians, then, it does make a vast difference what our teachings are..."
(J. Gresham Machen, Christianity and Liberalism, p 35-36)
This book is very relevant for today, especially in the face of those evangelicals who, along with some proponents of the emergent church, have come to embrace much of what early liberalism was putting forth.
This book is available here as a pdf, and here in Kindle format (free).