Based on the opinions I read on Amazon, and even in professional reviews, I believe there is a widespread confusion about what a biography can and cannot do. My sense is that this confusion is driven by the immense popularity of memoir and amplified by the twenty-first-century norm—driven by social media—of full disclosure.
The author of a memoir obviously has access to her innermost feelings as well as to every remembered detail of her life. Whether she chooses to recount them all is up to her, but the most valued memoirs are those that allow readers the most intimate and detailed perspective.
The author of a biography, on the other hand, must contend with several constraints. Most obviously, we cannot make up anything. We cannot simply imagine what is going on in our subject's mind. There must be available documentation.
If the subject is deceased and there is no letter, no diary entry, no official document, no media coverage, no remark by someone on the scene that has been preserved, our hands are tied. (A further complication: there might be an existing letter or a diary entry, but we have to be able to read it. Letters may be stored in the attic of someone unknown to us—or someone who refuses to give us access. A diary may have gone missing.)
There is also the matter of decorum and appropriateness. One of my rules as a biographer is that I do not enter the bedrooms of my subjects. Nor do I seek to discover what they might have told a therapist; such information is normally, and rightly, sealed off from public view. While I endeavor to understand my subjects' fears and obsessions, I am mindful that a biographer should not pose as a psychoanalyst.
The women I have written about made their mark as creative people. I write about them to interest readers in their accomplishments and to present a coherent narrative of their lives. Memoirs, for all their intimacy, often are sketchy about dates and places, and leave out areas their authors do not feel like discussing. A biography takes a larger, more carefully documented view.
That there is a place for both memoir and biography goes without saying. But it behooves the reader to understand these genres' different aims and constraints.