From the 18 year old who was in the same situation (even down to the age difference
) the best thing you can do is be there for her and trust her to come through it, she is young, it is likely she will see at some point over the next few years that he makes her feel more unhappy than happy, and she will gain the strength to get over that "can't" feeling she has about leaving. It took me three years
and I had mumsnet
- hang on in there.
Keep reassuring her that there is always a place for her at your house if it ever gets too much, whatever happens (even if you do end up having a big argument, I hope that doesn't happen :() and that you will always be there for her and help her with anything she needs. Don't offer her any coping strategies/solutions like trying to talk to him or ways of trying to manipulate him into a response as it just gives you false hope - it's best not to go on about him being an arse etc (however tempting) but reiterate that you can't change a person and people are who they are.
I would probably not give her the Lundy book. Keep it at your house, she might spot it, don't mention that you bought it for her. If she brings up in conversation things that he's done that she doesn't understand or things that he's done that have hurt her then you can say things like "That was really thoughtless of him, he hasn't thought about how you'd feel at all" etc, highlighting the actual way that he's let her down. Maybe when she gets into the questioning stage, is it me, or starts to recognise that he has an anger/control problem, then you could suggest the book, say you read it years ago or a friend gave it to you but you hadn't got around to reading it (since you don't have an angry or controlling man to read about any more!) but I think you have to wait until she brings up the topic of him being angry/controlling before he will accept either of those labels for him.
Being in an abusive relationship you go through such a complex amount of mind-states and if she says she "can't" stop seeing him, and she misses him when away etc then she's probably in the stage where she's infatuated bordering on obsessed, because he's convinced her that they are oh-so-special for each other, the rest of the world don't understand their love, he can't live without her, and any relationship she could have wouldn't compare with this one. At this stage it will be very hard for her to hear bad things about him, because any flaws in his character she will be almost proud of, because she's loving and understanding and helping somebody so damaged and that makes her a very special and rare person, because most people wouldn't understand, wouldn't care, or would label him a "bad person" without looking further to the person below all of that who she believes is the "real him". Also, she's at that prime age for seeing and multiplying flaws in herself to massive proportions, while putting others (especially boyfriends) on a pedestal and thinking that their bad points aren't so bad. In her mind, she probably thinks she has bad points which are equal to his and that they deserve each other. Every time she shows insecurity about anything you can believe he'll pick at this too in order to strengthen these beliefs of hers 
Once she gets to the point of realising that she's expending all of her emotional energy trying to calm the monster on the surface in order to reach the ~special and damaged soul~ beneath, and she realises that, actually, her life is pretty boring because it revolves around him while other people her age are going out, doing their own thing, following their own hopes and dreams, etc, then you'll be more able to get through to her, but at the moment she's got more than enough energy to keep on giving and she's all consumed with him and hasn't noticed that he is taking away all of her. She will get there, she is still the same person inside that she always was. She'll just have to find herself again, it's just that you have to get pretty lost to realise that IME. Also, although it's going to be very hard for you to watch, you might find it reassuring to know that it's quite rare for women in her situation to commit suicide especially if they know that there are options (whether they feel able to take those options or not) because she's not suffering from depression, although she might be displaying several of the same symptoms.
Good luck, I hope it does not take too long for her to see the light. xx