There's lots - Plutarch wrote a life of Caesar, and tells us he writes his lives less for historical accuracy than for giving an impression of the characters of his subjects. Suetonius has all the gossip. Cicero knew him and wrote a lot about him to his friends, and I think we even have some correspondence between Caesar and Cicero. Sallust knew him and wrote something of a eulogy in his monograph on the Catilinarian Conspiracy. And you could infer personality from how he handled his public business.

If it's not as written about in secondary literature, it may be because that kind of thing is rife for personal opinion and unconfirmable anecdotes. Did Caesar really say he would prefer to be chief of a hill tribe than no one in Rome? Or is that kind of early sign of ambition projected on to stories about his early life because the author knows what's going to happen? (Hint: it's probably the second one!)

So there is lots of information, but deciding what to believe is a lot harder than deciding of such and such a law got passed in his second or third consulship.

As an addendum to /u/LegalAction, we have several letters of Cicero (to his friends, especially Atticus) in which he discusses Caesar upon the even of the latter's invasion of Italia in January 49 BCE. I've excerpted part of one below, with my notes in square brackets [] (ad Atticus VII.7):

We should have resisted [Caesar] when he was weak, and that would have been easy. Now we are confronted by eleven legions, as much cavalry as he wants [conscripted from the Gauls], the Transpadani, the city rabble, all these tribunes, a rising generation corrupted as we see, and a leader of such influence and audacity! With such a man we must either fight a pitched battle, or admit his candidature in virtue of the law [Caesar wished to stand for high political office without being physically present in the city, and the Senate refused]. Atticus, you would say "Fight, rather than be a slave." To what end? To be proscribed [put on a kill list], if beaten; to be a slave after all, if victorious? "What do you mean to do, then?" say you. Well, just what animals do, who when scattered follow the flocks of their own kind. As an ox follows a herd, so shall I follow the boni [conservative senators] or whoever are said to be boni, even if they take a disastrous course. What the best course is in this unfortunate dilemma I see clearly. For no one can be certain of the result when once we come to fighting: but everyone is certain that, if the boni are beaten, this man [Caesar] will not be more merciful than Cinna in the massacre of the nobility, nor less rapacious than Sulla in confiscating the property of the rich.