Other useful books:

My Puppy's A Poo Magician! A Story About a
Puppy Who Is Scared to Poo ​by Melissa Yapp
Amazon

The Ins and Outs of Poop
by Dr Thomas R. DuHamel
Amazon

A Boy Like You: A Story for Boys Who Soil
​by James Parkin
UK Edition: ​Amazon 
USA Edition: Amazon I eric.org.uk

Constipation, Withholding and Your Child 
by Anthony Cohn
Amazon

A Girl Like You: A Story for Girls Who Soil
​by James Parkin
UK Edition: ​Amazon
​USA Edition: Amazon I eric.org.uk  

Scared to Poop: A Guide to Overcoming Constipation
and Stool Withholding by Kathleen Diehl
Amazon