Detail of Alexander in his victory over the Persian king, mosaic, circa 100 B.C.E. House of the Faun, Pompei (Source:Wikimedia Commons, public domain)
Companions and handbooks are very helpful kinds of sources. They are edited by leading scholars in the particular areas of research, and those editors commission researchers to write individual essays on the subjects which are their specialties. As a result, students get a much better understanding of the issues involved in a particular subject area, since the authors go beyond factual summaries to analysis of differing arguments in the field.
Additional relevant material may be found in the series listed below when you look by Classical author, literary genre, or topic.
Blackwell Companions - 65 Titles
Brill Companions - 35 Titles
Cambridge Companions - 44 Titles