Secondary accounts include contemporary diarists who were not present at the events and later books written from primary material. A book built mainly on them filters its argument through earlier writers' choices and analysis. It may therefore be examining what others have said about an event more than reconstructing the event directly. That remains worthwhile, but it is a different kind of historical work. Reading the source list first tells the reader which kind of contribution to expect.
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Secondary accounts add another layer of analysis
A bibliography dominated by secondary accounts can still support worthwhile history, but it shows that the author's interpretation passes through earlier analyses.