I'll nominate A Memoir of Jane Austen by James Edward Austen-Leigh. James was the nephew of Jane Austen "The Memoir remains the prime authority for her life and continues to inform all subsequent accounts."
It is free from Project Gutenberg and often included in compilations of Austen's works. If you wish to pay a not-too-unreasonable price then the excellent A Memoir of Jane Austen: and Other Family Recollections (Oxford World's Classics) is available on Kindle. My nomination, however, is for the Memoir.
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