Looking for a good biography is much like looking for a scholarly history book. A good one will typically get published by a good publisher ( not, say, the author herself) , be cited by other scholars, and have a good list of sources .

There are some older biographies that will never actually be obsolete. Boswell's Life of Johnson used the notes that Boswell took while hanging out and traveling with Samuel Johnson, and all later biographers cite it. On the other hand, there's often a tendency for any author to say something admirable about their subjects, or at least to make a case for them being more important, significant. Both admiration and importance can be subject to change. For admiration, Douglas Southall Freeman's massive 4-volume biography of Robert E. Lee, done in the 1930's, was long considered a model of exhaustive, scholarly research and good writing. But Freeman, a Virginian, had a very Virginia-centric view of the Civil War, and was anchored pretty firmly in the Lost Cause school of Civil War history. His portrait of a heroic, virtuous Lee has held up about as well as the Lee statue on Monument Avenue in Richmond, that Freeman used to regularly salute when he drove by. For importance, the bookshelf is full of works on George Washington, and there's no doubt about his significance. But Erica Armstrong Dunbar's recent biography of one of his escaped slaves, Ona Judge, has been quite well-received. A hundred years ago it almost certainly would not have gotten more than a short article in a small historical journal- possibly merely rated a footnote.

Dunbar, E. A. (2017). Never Caught: The Washingtons’ Relentless Pursuit of Their Runaway Slave, Ona Judge. 37Ink.