(The Disney+ series "A Small Light" is loosely based on the events in this book.)
Miep was Otto Frank's employee, close friend of the Franks, and helped hide the family during the time that Anne wrote her diary. (She was the person finding the Frank's food.) After the Franks were arrested, Miep rescued the diary and then gave it to Otto Frank when he learned that Anne didn't survive the concentration camps.
The book gives significantly more context than Anne's diary regarding who the Franks were, and the struggles they encountered. Not only is it a vital companion to Anne Frank's diary, Miep's account of Amsterdam during WWII describes in detail what life was like under the Nazi regime. She gives a lot more of the "who, what, where, why, and how," that Anne's diary omits. (Remember, Anne was a teenager who, initially, was writing for herself and not for a global audience.)
The first section of the book describes some of Miep's background, how she met the Franks, their relationship, and the history of the Franks. The second section describes the period where she helped hide the Franks, and often gives an alternate perspective to events that Anne wrote about. The third section describes the Frank's arrest and the famine that happened in Amsterdam before the Germans surrendered.
I read the book because some events in "A Small Light" were a little difficult to believe. And, yes, the events in "A Small Light" that I didn't believe weren't part of the book.
Edit: https://www.amazon.com/Anne-Frank-Remembered-Helped-Family-e...
Edit 2: I also recently re-read Anne Frank's diary. As an adult with children, it provides a strong reminder of how teenagers' minds work. It also shows that even our heros were immature as youngsters. In Miep's afterword, she points out that she was really hurt at the way that Anne described some of the people in hiding.