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
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.