Below are some of my favourite books and websites on this topic. 

Young Minds -

Self-harm (youngminds.org.uk)

NSPCC - Preventing Child Self-Harm & Keep Them Safe | NSPCC 

NFA-  Understanding Self-Harming Behaviour in Children (nfa.co.uk)

These recommended books are linked to Amazon UK and may generate a commission which goes towards the costs of running this site.