‘Developing Mental Toughness - Coaching strategies to improve performance, resilience and wellbeing’ by Strycharczyk & Clough (£29.99 on Amazon)
This is a great introductory book for any coach beginning to understand mental toughness and how it is best utilised.
I personally have used this with a couple of athletes this year alone. Where I simply ask them to read chapter 3 and make some notes that we can then discuss together informally. I found this really engages the athlete with the theory and helps the coach to understand a little more about what makes their athlete tick.