The Wars of the Jews, by Flavius Josephus.
Surprisingly enough, this is a rip-snorter. If you want to learn about Roman siege tactics, this is the book for you.
A couple of warnings:
When Josephus refers to a rebel leader named “Josephus” in the text, he refers to himself. He was initially one of the Jewish rebels against Rome, then was captured by the Romans, and ultimately concluded that the rebels were, as my old boss would have said, “in error.”
Also, the translator wastes much energy in trying to align Josephus’s words with the Christian scriptures, but, fortunately, such is confined to the footnotes.
Also, if you can, kick a few bucks to Project Gutenberg. It is one of the best things on the inner webs.