Biography is the form that allows readers to spend the most time with a single mind in depth. A good biography — one that takes seriously the subject's inner life, historical context, and the gap between their self-understanding and external reality — is among the most intellectually satisfying reading experiences available.
The biographies that endure share a common quality: the biographer maintained intellectual independence from their subject. Hagiography — the biography as tribute — produces books that are less useful because they omit the information that would allow a reader to understand the subject's actual choices and errors. The best biographies are written by writers who admire but are not uncritical of their subjects.
Robert Caro's multi-volume life of Lyndon B. Johnson is the most ambitious biography project in American letters. Caro has spent 50 years on the project and it remains unfinished. The first volume, The Path to Power, establishes how a specific regional political environment shaped a young man's understanding of power. By volume three (Master of the Senate), the reader understands American political structure more deeply than most political science texts provide.
For shorter, more accessible biography: The Innovators by Walter Isaacson (the collective biography of computing pioneers), The Wright Brothers by David McCullough (the technical and human story of powered flight), and The Emperor of All Maladies by Siddhartha Mukherjee (the biography of cancer as a disease) are excellent entry points.
For biography as literary form: the Lives of the Artists series edited by Giorgio Vasari, first published in 1550, established the biography as a narrative of creative personality rather than just chronological achievement. Everything written since operates in its shadow.