It is a real shock when it happens and it is important she doesnt feel like what she is doing is making life worse for everyone. You may not comes to terms with what is happening very quickly but try around her to be normal and calm and consistent and quite practical , controlling extreme emotions .
Its hard to recommend books, and when I was going through my own DDs severe depression, a therapist suggested that there is no 'normal' when dealing with teen mental health issues so its hard to categorise into chapters. She may well have no idea why she feels as she does, and thats why expert help (not emotionally involved too) is important and her specific thoughts and symptoms assessed and she is given specific tools to cope. Is it stopping her from being at school and on top of her work ? Is she sleeping ? is she eating?
Young Minds website is helpful for parents and you can even ask for a callback from a mental health professional to get advice how to approach things.
You really can help things get better as other posters advise Now she has admitted it, You should be honest with the school about the level of SH (they will be bound by confidentiality) and they will have come across it before . you could ask to speak to the counsellor so you have confidence they have experience (schools differ) or find a private psychotherapist for an assessment if Camhs take too long.
Explain to her if you can that talking with someone is not to make her stop SH, but to help her understand why she does it, and how she can be helped to feel differently so she doesn't feel as much need to do it. Do keep telling her your love her, support her, and whatever she is feeling, you will be there for her. You may not understand what she is going through, but you and her Dad will be there no matter what. When a teen is low, they need to hear you being strong and in control maybe more than you imagine. Don 't pressurise her to talk to you, but give her confidence that many other young people go through these difficult times, and things will get better with the right support and help.
She may respond swiftly to help , but I advise professional assessment as there are many of us on here, whose teens have needed ultimately much more support and help, much patience and more time than anyone imagines. its a really positive thing you know what is happening and she is talking to you before the SH has escalated further. I hope something in here is helpful an wish you all well. .
Yegodsand little fishes gives good advice and I havent seen the animals reference before. It's true.