J.V. Fesko has written a very helpful little booklet explaining the doctrine of justification by faith alone. At only 32 pages this is a very accessible overview of the doctrine 'upon which the church stands or falls' (Alsted) and 'the main hinge on which religion turns' (Calvin). Fesko gives a clear explanation of the Biblical teaching, focusing on the Garden of Eden, Abraham, and the life, death and resurrection of Christ. He also deals with the objections raised by antinomianism and legalism.
Overall, this is a sound introduction which could be used with inquirers or believers. It is quick and easy to read, but packed full of solid content.
Book details: J.V. Fesko, What is Justification by Faith Alone? (Philipsburg: P&R, 2008)