Sallust, Florus, and Velleius Paterculus, literally translated, with copious notes and a general index. By the Rev. John Selby Watson. New York: Harper & Brothers, published 1859.
Sallust on wikipedia.
Information on Florus and Velleius Paterculus from Livius.org.
The translation of Sallust by Watson is common on the web, not least because an edition of it exists at Project Gutenberg. This book, however, also includes four letters (two to Caesar) as well as the histories of Florus and Velleius Paterculus, none of which appear at PG (as far as I know).
Why do I keep thinking of Hogwarts as I type this?