The Scarred Soul by Tracey Alderman is a really good self help book for overcoming self injury. It contains alternative coping skills, reasons why people self harm, how to talk to someone who self harms, and worksheet activities.
Crosses by Shelley Stoehr is more of a book for teens about two friends that cut themselves.
Skin Game by Caroline Kettlewell is an autobiography. She shares her journey through healing of self injury. I recommend this book if you want to understand the struggles of it all.
But all three are good reads.