The most important source after Polybius is Diodorus Siculus who wrote his Bibliotheca historica between 60 and 30 BCE and reproduced some important earlier sources such as Hieronymus, but his account of the Hellenistic period breaks off after the battle of Ipsus (301). Another important source, Plutarch's (c.50—c.120) Parallel Lives though more preoccupied with issues of personal character and morality, outlines the history of important Hellenistic figures. Appian of Alexandria (late first century CE-before 165 CE) wrote a history of the Roman empire that includes information of some Hellenistic kingdoms.
What is next on the list of significance?
Diodorus