The best book I can recommend to you on Simon de Montfort is the song of Simon de Montfort by Sophie Therese Ambler, it is brilliant on his personality and influences.

I would also recommend the First English Revolution by Adrian Jobson, if you’re looking for a narrative history of the Barons War. His relationship with Henry III is explored by David Carpenter in the Reign of Henry III and Henry III volume 1.

R F Treharne’s essays are a great place to explore various aspects of Simon de Montfort.

Maurice Powicke’s Thirteenth Century, Michael Prestwick’s English Politics in the Thirteenth Century and Barbara Harvey’s Twelfth and Thirteenth Centuries, provide excellent coverage of Simon in a wider context.

Paul Brand’s King’s, Barons and Justices is an absolutely brilliant place to turn for legal and constitutional implementation of the Montfortian reforms.