The following are resources from Ed Decker's "Saint's Alive" ministry website with respect to understanding Freemasonry.
Freemasonry: An Introduction to Heresy
Freemasonry Resource Files
Reasoning from the Scriptures with Masons
Like all of Ron Rhodes books, this 120 page book is very easy to read and easy to use in witnessing to Freemasons or to those who may be unaware of what Freemasonry really is and think that it is only a social group.
The Deadly Deception
Jim Shaw is a former top leader in Freemasonry, having achieved the 33rd degree, and thus can speak very knowledgeably about the organization.