Like with most historical fiction, I try to take the facts I don't independently know with a grain of salt, assuming most internal thoughts and private conversations (at least) are almost entirely fictionalized.

Luckily, quite a few of Cicero's own writings survive, so I assume a lot of the content is taken from his letters with Atticus, various of his speeches and essays, etc. And the events surrounding Julius Caesar's rise and fall are overall some of the most well-documented of the entire ancient world, so less would probably need to be fabricated than you might think.

Having not read most of the ancient sources myself (Plutarch's "Lives" and some excerpts from Tacitus and Suetonius notwithstanding), I can't provide a proper historian's review, but nothing struck me as particularly implausible. Near as I can tell, Harris sticks relatively close to the traditional historical narrative. He doesn't really make any sort of controversial statements regarding known events that I'm aware of.