Westminster bookstore is offering J. Gresham Machen’s Christianity and Liberalism for 6.50 (+S&H). This is a must read book.
Machen’s thesis is reflected in the title. Liberalism is not Christianity; it is another religion. This gets at the root of the Fundamentalist objection to ecumenical endeavors with with liberals.
For further Machen resources, see John Piper’s biographical sermon and the biographies by Hart and Nichols (Hart’s is the more detailed, but I profited more from Nichols).