As her nephew, he was in a position to use is own recollections of the author, as well as verbal accounts from his near relatives. Very few of her letters and manuscripts have survived; Jane Austen was a family-oriented person; hence family was a very important source of information about her. As well, he has also written about manners and customs in her time, which had changed even by the time this memoir was written, but not so long before as to be forgotten by older contemporaries of the author.
The author has also tried to provide a fairly balanced opinion about his aunt, while still calling it a memoir and not a biography.
I found it very interesting reading.