I don't have any books to suggest because frankly, I have never read any that I really agreed with. But I will offer something here that may help you and your sister a bit.
First things first, supposing that your sister does believe in the Lord, and does trust the Lord, which I assume you believe she does as well, I have personal experience with the issue of fear, very similar to what your sister describes.
I experienced this growing up a lot, from about the age of 3 1/2 until I was about 15. Now I must say, I have very good memory, and at the age of 3 1/2 I was very aware of what was going on, I still remember back all the way to age 2, but on to the subject.
The devil, or demons, or both have always my whole life long attacked me with nightmarish dreams, even when I was 3 or 4, I would have dreams about things I had never seen before, horrible acts of violence, and monsters and the like, we did not have tv, and my mom did not let us read scary things like that, so the only source for it was demonic.
Anyhow, by the time I was seven, I had developed a very serious fear issue, I was afraid of the dark, not because of monsters, but because I had seen demons in my dreams, and I knew that demons were real. So I was terrified that I might see one while I was awake, and I was terrified that I might see one while I was asleep.
As this went on, it developed to the point that I worried about my family, and I would just sit awake in bed until 2, 3, or even later in the morning, because I couldn't sleep for fear of what might happen to me or someone else. I would hear a noise and it would make it worse, if I ate anything, like you said, there was a lump, and it would make me sick to my stomach. I was even scared to breathe for fear that it might be heard.
What I did not know, was that I was giving in to a spirit of fear, and allowing it to rule and control my life.
What I did not know is that that is sinful to let fear rule my life.
When I realized that, I had to repent of letting fear rule me like that, and when I did, it broke the power of fear in my life. Now let me say this, I was not instantly unafraid any more, but it's power was broken, I wasn't bound up in it anymore, and I found that I had power in prayer over my emotions, so that I could sleep again.
It was a very slow process, and I dealt with it for a long time, but I still deal with the dreams.
Now, I still don't like being alone on a dark night, it is a little creepy, for almost anyone it is, but that is natural. But what she is going through sounds very similar to what I went through.
While they are not identical, they are similar, and I would be willing to bet that she is, or has experienced dreams of some kind that sparked a fear in her, and then it built upon that foundation, and she now allows it to control her.
I don't want to sound harsh at all, but fear or worry is a sin, and it is dangerous, it causes people to do unnatural things. It is something that she needs to repent of, and then work through it in prayer.
I had to go before the Lord every time I was afraid, or worrying after that and just say, "Jesus, I need you right now to protect me, Jesus cover me with protection, calm my thoughts, calm my mind, calm my emotions, give me the peace that passes all understanding, blind my mind to fear, and doubt, and worry, and Jesus, help me go to sleep please."
I didn't pray the same thing every time, but I had to take it before the Lord every day, sometimes many times in a day, and God was faithful to deliver me from it.
Well, I hope this helps, sorry if anything sounded harsh, no intention of that at all.