You could always start with the ancient sources, like Plutarch's Parallel lives and Suetonius' life of the divine Julius.

There's also Nicolaus of Damascus' account of the assassination, which is considered one of the more reliable ones. Nicolaus did not witness the assassination, but had the chance to talk with witnesses.

You could also check Livy's Ab Urbe Condita, I'm not sure, but I believe he wrote something about it. Marcus Tullius Cicero, who only survived Caesar for one year, is also a good source, yet a bit subjective on the fall of the republic and Caesar.

These are the ones I can think of right now. Most of these books you can find online, here!.

There is also this one! which I found entertaining to read. I hope this helped! If you have more questions, just ask away!