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By the time he wrote "De Oratore" near the end of his career, Cicero was a mature veteran of many courtroom encounters, and he had become somewhat ashamed of "De Inventione." He shouldn't have been. Although "De Oratore" reveals him to be a master trial lawyer whose deep understanding of human persuasion was augmented by years of experience, his earlier work reveals him to be a talented rookie who had a good academic knowledge of rhetoric.
The basic theory expounded in "De Inventione" remains in "De Oratore," but the seasoning he acquired from decades as an advocate makes "De Oratore" a much more polished exposition of the praxis of rhetoric. If you only have time to read one book, read "De Oratore," but if you want to plumb the depths of Cicero's mind on the subject of rhetoric, read both. If you read both, you can follow Cicero as he matures from callow but talented tyro to the greatest advocate of his age.
To the Phillipic itself, it is a must-read for anybody studying the late Roman republic. Cicero's vile attack upon Antony is world famous and has been passed down through history. Seperating fact from fiction, with all the exaggerations in between, is quite a task, but despite the fact that a good half of what Cicero claims simply was not true, it is an invaluable source for this period in history.
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The book itself (The Nature of the Gods), on the other hand, is great. It was written (of course) by Cicero, who was one of the greatest writers, statesmen, and philophers of his time and is the one actually brought many of the philosophical term from Greek into Latin, which made it possible for many Romans to discuss philosophy, whereas they could not before. It gives a glimpse of the leading philosophies of his day: Epicureanism, Stoicism, etc. It's definitely worth reading (a different translation). I would suggest the Penguin edition.
To the publisher's credit, there's excellent biographical sketch of Cicero in the front of the book, which is helpful and concise.
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I find the letters to his secretary the most interesting because Cicero shows himself here as a caring human being ( in contrast to his cold and formal attitude towards his wife ).
In general though The Letters have a tendency to rhetorics and are therefore often long-winded and sometimes a little boring.