Question:
"What do you suggest I read to learn to better witness to Mormons?"
Answer:

There are many helpful resources elsewhere on the web as well. James White's "Verse Memorization System: Verses Relevant to Sharing the Gospel with Mormons" is very useful. You could learn a lot from web sites such as Mormonism Research Ministry, the Institute for Religious Research's Mormons in Transition, the Mormonism section of 4Truth.net, and others.
If you would like to learn more about Mormonism in general and how their beliefs are different from historic Christianity, I suggest checking out Is the Mormon My Brother?, Mormon America, and Mormonism 101. And if you are firm in your faith and want to dig deep, nothing can replace actually reading Mormon sources themselves. I would start with Gospel Principles.
I could continue, but I'm afraid that this list has already become too long. Feel free to ask me additional questions on witnessing to Mormons. Praise God for your desire to bring the light of the true Jesus Christ to Latter-day Saints!