A lot of people are confused into thinking that reading the Book of Mormon in its entirety is necessary to judge whether it is influenced by demons.
The following analogy might help.
Suppose you learn that a husband has written a very nice, lengthy letter to his wife. But then you learn that the husband has had multiple affairs, is manipulative, a liar, and physically abusive. Moreover, you learn that he was doing all this at the same time that the letter was written. Do you need to read the full letter to know whether it had negative influences?
No, you do not.
In a conversation about influence or inspiration of a given work, what’s primarily relevant is the proximate context.
Now, suppose that you decide to confront the husband. You point out the fact that he was unfaithful, manipulative, abusive, and so on when he wrote the letter.
Suppose he responds by asking, “But did you read the whole thing?”
Most people can see how silly of a requirement this is in this context. You don’t need to read the full letter to know that there’s something deeper going on.