I'd be remiss if you I didn't recommend Graves's I, Cladius.

There's always Robert Harris' novels about Cicero: Imperium, followed by Lustrum. A third novel (I think it's going to be called Dictator) is coming eventually. Lustrum's title in the US is Conspirata--presumably US publishers figured that Americans wouldn't know what a lustrum is so they gave it a nonsensical name that sounds like it should mean "conspiracy," but doesn't (a conspiracy is a coniuratio or occasionally a conspiratio. Conspirata means either "conspiring woman" or "multiple neuter conspirators," i.e. lifeless objects all conspiring at once somehow). I've never read them but a member of my department who specializes on Cicero highly recommended them. The only problem is that he's made a problem for himself. Harris has said that he intends to take the work all the way down to Cicero's death, but his first two novels only bring us up to his exile in 58. The first novel begins around 79, when Cicero went to study under Apollonius Molon, so the two novels cover 21 years (and they're fairly long novels, both of them). The third will somehow have to cover the remaining 15 years without becoming absurdly long--not an impossible task, considering that the majority of Cicero's political work took place before his exile and immediately after Caesar's death, but this last novel will have to include the entirety of Cicero's maneuvering with Pompey, the whole civil war, Caesar's dictatorship, Cicero's political campaign against Antony, as well as the composition of most of his philosophical works (which were largely published after 56, when Luca forced him to support the triumvirs and he largely retired from public life). It's a tall order

I'm no classicist, so I can't speak to the accuracy of these, but as Spoonfeedme said, definitely look at Graves (although Claudius the God, the relatively lesser-known sequel to I, Claudius, isn't comparable to the first). If anything, I, Claudius is a classic in its own right. Perhaps also check out the late Australian author Colleen Mccullough's Masters of Rome series. They're very hefty, but if I remember correctly, her works went largely uncriticised by classical historians.

Contrary to other posters in this thread I'd suggest you stay as far away from I, Claudius as you possibly can. This is part of a reply I wrote to another user a little while ago, concerning that particular series:

Admittedly I havn't watched the series until the end (I have the DVD box in my bookshelf but I can't bring myself to finishing it), because I got so annoyed with the portrayal of the characters in the series. The overall problem with I, Claudius is that it's written by a 19th century classicist, which in itself isn't terribly bad, but means that he's reading those sources in a way that a modern historian wouldn't do. Because of that, and because it's based on Suetonius' De Vita Caesare (The Lives of Caesars), everything in the series has a very anti-imperial family angle.

A straight answer to your question is that it's not very fictionalized at all. If you were to read Suetonius you'd find yourself right at home in what is described. However, Suetonius was a senator and as such his power diminished with a powerful imperial family like the Julio-Claudians. To simplify, what Suetonius writes, while not blatant lies, is often angled in a way that makes the imperial family look bad. He also isn't very good at sifting through rumours, which is why Livia is such a, well, bitch in the series.

If you were to watch I, Claudius, I'd recommend that you watch the HBO series Rome as well and compare the portrayal of Octavian and Livia to the one in I, Claudius. It is quite different and in my opinion a lot closer to the truth. The actors who played those two characters in season 2 really played the roles well. The whole Rome series is a pretty good work of historical fiction on it's own, but obviously isn't reading material.

I would have recommended the Emperor series to you, but you seem to have found them on your own! They do seemingly get quite bad reviews - I've seen a few on here for instance, and there is one on Amazon. However, I believe it's important to remember that it is historical fiction we're talking about and Conn Iggulden is a very engaging writer who manages to piece together the full story of Julius Caesar while not bending the truth too much. There are historical issues with the series of course (eg. Brutus and Caesar would not have played together in the woods at age 6), but that's expected from historical fiction. Another similar author that you may want to take a look at is Ben Kane and his book Hannibal: Enemy of Rome. It's written in a similar style, some largely made up parts which engages the reader in a story, but a story that still follows the recorded historical process. The series is in 2 parts I believe (I've only read the first book so far) and portrays the Second Punic War, with alternating storylines between a young highborne Carthagian and a young equestrian Roman.