This book — Muhammad: Man and Prophet, by Adil Salahi — is hands down my favourite in the English language. It's just the right level of detail and it's written in good, simple language. I'm going through it with my niece and, in the process, I'm creating a set of slides for each chapter which captures the key points. You can find links below to these slides.
Chapter 3 – From Youth to Maturity (Pre-Prophethood)