Companions and Handbooks are groups of essays by different authors on a topic in a single volume. Though each essay/chapter within a work can be quite specific, the work in its entirety gives an overview. Companions and Handbooks serve as excellent starting points for a research paper.

eBooks list - but many additional print books are available too

• A Companion to the Roman Republic 
• A companion to Greek democracy and the Roman republic 
• A Companion to Plutarch
• ​A Companion to Sport and Spectacle in Greek and Roman Antiquity
• A Cambridge Companion to Cicero
• Brill's Companion to Cicero
• A Companion to the Archaeology of the Roman Republic
• The Oxford Handbook of Greek and Roman Coinage
• A Companion to Greek and Roman Sexualities
• The Cambridge Companion to the Roman Historians
• A Companion to the Roman Army
• The Cambridge Companion to Greek and Roman Theatre
• The Oxford Handbook of Roman Epigraphy
• Handbook to Roman Legionary Fortresses
• The Oxford Handbook of Roman Studies
• The Oxford Handbook of Engineering and Technology in the Classical World
• The Oxford Handbook of Warfare in the Classical World
• The Oxford Handbook of Roman Epigraphy
• ​The Oxford Handbook of Social Relations in the Roman World
• The Oxford Handbook of Jewish Daily Life in Roman Palestine 

Legionary shield blazons from Trajan's Column.

Classics - Oxford Bibliographies combines the best features of an annotated bibliography and a high-level encyclopedia. It provides expert recommendations on the best works available – whether it be a chapter, a book, a journal article, a website, an archive, or data set. 

The Cambridge Histories are written by reknown scholars in the field. Each chapter in a volume can be read independently, and used as a paper source.

•  The Cambridge History of Greek and Roman Warfare Volume 1: Greece, The Hellenistic World and the Rise of Rome

• The Cambridge History of Greek and Roman Warfare Volume 2: Rome from the Late Republic to the Late Empire

• The Cambridge Ancient History Volume 7, Part 2: The Rise of Rome to 220 BC

• The Cambridge Ancient History Volume 8: Rome and the Mediterranean to 133 BC

• The Cambridge Ancient History Volume 9: The Last Age of the Roman Republic,  146–43 BC