These are a selection of the up-to-date Secondary Sources (as of the 1770’s-1780’s) used by Gibbon when he composed each volume in his 6 volume Decline and Fall of the Roman Empire.
Secondary Sources are histories which quote/use primary sources, and summarize and explain (and create) a history or a historical narrative.
A Primary Source is a document from the past which is of historical significance (ex. law books, ancient contracts, etc) from the actual period of time being studied (ie contemporaneous). What is a secondary source for most modern historians (ie histories) is usually the only primary source we ancient historians have available – so our definition of primary sources usually includes ancient historians/histories as well. A definition of Secondary Source from Wiki here.