The Girl Empress by Amy Mantravadi

The history books remember her as Empress Mathilda, but her path to sovereignty began when she was just a girl named Maud. Engaged, estranged, and crowned by the age of twelve, this is her story . . .

As the firstborn legitimate child of King Henry I of England, Princess Maud is faced with the fiercest crisis of her eight-year-old life when she learns that she will be sent to Germany to marry the Holy Roman Emperor. To make matters worse, her husband-to-be is in the midst of a disagreement with the Pope, and the threat of civil war continuously rages. Thrust into the middle of the greatest political controversy in Europe, Maud must learn to navigate the turbulent political waters while also managing her own transition from girl to woman.

Students of history will know the ending: Maud will successfully become Empress of the Holy Roman Empire, Queen of Germany, Queen of Italy, and the sole legitimate heir to the English throne. But The Girl Empress invites readers to join Maud on the journey of a lifetime, experiencing the depths of her hopes and dreams, her anxieties and strengths, her successes and failures. The history books haven’t done her justice.

Matilda by Catherine Hanley

From Yale University Press:

Matilda was a daughter, wife, and mother. But she was also empress, heir to the English crown—the first woman ever to hold the position—and an able military general.

This new biography explores Matilda’s achievements as military and political leader, and sets her life and career in full context. Catherine Hanley provides fresh insight into Matilda's campaign to claim the title of queen, her approach to allied kingdoms and rival rulers, and her role in the succession crisis. Hanley highlights how Matilda fought for the throne, and argues that although she never sat on it herself her reward was to see her son become king. Extraordinarily, her line has continued through every single monarch of England or Britain from that time to the present day.  Publication date: March 2019

It should make a good companion to "The Empress Matilda" by Marjorie Chibnall