Inspired by the resources found at the end of each chapter of Walter Martin's excellent Kingdom of the Cults book, here is a good launching point for doing deeper research and study on some of the major cults. This list is by no means comprehensive, but it does represent some of the main resources recommended in Martin's book (reviewed here). Share your suggestions in the comments.

Jehovah's Witnesses
Jehovah's Witnesses by Robert Bowman
The 10 Most Important Things You Can Say to a Jehovah's Witness by Ron Rhodes
Reasoning from the Scriptures with the Jehovah's Witnesses by Ron Rhodes
Jehovah's Witnesses Answered Verse by Verse by David A. Reed

Christian Science
God's Perfect Child: Living and Dying in the Christian Science Church by Caroline Fraser
Christian Science: Abuse, Neglect, and Mind Control by Linda S. Kramer
The Christian Science Myth by Walter Martin
Christian Science by Mark Twain

LDS, Mormons
Mormonism 101: Examining the Religion of the Latter-day Saints by Johnson & McKeever
Mormons Answered Verse by Verse by Reed & Farkas
The Changing World of Mormonism by Jerald & Sandra Tanner
Mormonism by Kurt Van Gorden
Is the Mormon My Brother? by James White
Reasoning from the Scriptures with the Mormons by Ron Rhodes

Spiritism
Occult Bondage and Deliverance by Kurt Koch
Unmasking the New Age by Douglas Groothuis

Baha'i
What is the Baha'i Faith? by William McElwee Miller

Unitarian Universalism
UU Strategies for Sharing Faith and Growing Congregations by Scott Alexander
Unitarian Universalism by Alan Gomes
History of Unitarianism by Earl Morse Wilbur

Scientology
A Piece of Blue Sky: Scientology, Dianetics and L. Ron Hubbard Exposed by Jon Atack
L. Ron Hubbard: Messiah or Madman by Corydon & Ambry
The Deceivers by McDowell & Hostetler
Bare Faced Messiah: The True Story of L. Ron Hubbard by Russell Miller
Misguiding Lights? by Stephen Miller

Unification Church
Sun Myung Moon and the Unification Church by James Bjornstad
Heavenly Deception by Chris Elkins
My Life in the Reverend Sun Myung Moon's Family by Nansook Hong
Unification Church by J. Isamu Yamamoto