A historian’s later account of an event usually draws on earlier documents, objects and other research. It is a secondary source because it interprets that evidence. It can provide context and compare records that a reader could not easily assemble alone.
The distinction depends partly on the research question. A history book can itself become a primary source if you are studying how people wrote history at the time it was published. Neither label guarantees accuracy. Ask who created the source, for what purpose and on what evidence.