Which do you prefer? Biographies written about someone? Or Autobiographies written by the actual person (and/or ghost-writer)?
Biographies are fine. Autobiographies are better. But I'd really rather read a memoir.
Biographies and autobiographies are usually broad in scope, sometimes even covering the subject's whole life, whereas a memoir often centers on a particular aspect or theme.
Autobiographies and memoirs are fascinating because they reveal so much more than the plot the person chooses to tell. I read and wonder, why this particular focus? How do they select what to include? what to leave out? How are they shading and shaping readers' perceptions? How much is true and how much is wishful thinking? or morphed memories? Are they revealing themselves? or recreating themselves?
See what I mean?