I read this nearly ten years ago, so the memory is fuzzy. Sheldon, Intelligence Activities in Ancient Rome: Trust in the Gods but Verify. You can see from the Reagan reference in the title it comes from a particular time and political space.

If I recall correctly, the Caesar cipher is effective if you are not very good with the language you are reading. On the other hand, in practice even if you send a message in cipher, the guy you're sending it to needs to be able to read it, and maybe in quick order. You don't want him doing too much work figuring out what you want.

Unfortunately my brief survey of the sources doesn't reveal Caesar talking about the cipher himself; the source for this seems to be Suetonius and he apparently solved it because he can say you substitute D for A. My guess is that Caesar meant for it to be minimally secure. He wrote lots, so we should assume a fair number of people (including Suetonius, eventually, years later) had the pre-arranged key, or at least knew the principle if they didn't have the key for whatever particular message they got their hands on. On the other hand, if you couldn't read or weren't that familiar with Latin or Greek, piecing that message together might take a fair while.

This is a little bit off topic, but a really creative method of encoding was the scytale - this is a contraption in which letters were written in a thin strip in order, and when rapped around a wand the message appeared. You have to know the correct diameter of the wand in order to get the correct message.

Here is a link to a brief review of Sheldon's book.