If you want to evangelize the Jehovah’s Witnesses, then
this book may be useful. Though it is written from a Protestant perspective, its aim is to refute the book called
What Does the Bible Really Teach? which JWs use in their “Bible studies” with unbelievers. The author has specifically tried to ask questions that the JWs aren’t prepared to answer.
With Mormons, they often claim to have a “testimony” from God that the Book of Mormon is true. They will tell you that they “know” it is true because they asked God if the BoM was true and they felt a ‘burning in the bosom’. However, the Book of Mormon actually says:
It literally says to ask if those things are
not true, and so the ‘burning in the bosom’ could mean that the BoM is not true.
Another interesting tidbit about Mormonism is the Book of Abraham. Joseph Smith got his hands on an ancient Egyptian papyrus and “translated” it the same way he did the Book of Mormon. He said it was about Abraham. Since no-one at the time could read hieroglyphics, they couldn’t prove Smith wrong. The papyrus disappeared but then turned up again in the 1960s. Egyptologists translated it and it turned out to be a funerary text and not at all what Smith claimed. Since Smith claimed to translate the BoM in the same way, it doesn’t look good for him.