The primary source for Shakespeare's Julius Caesar is Plutarch's Lives. He concentrates on the Caesar, Mark Antony, and Brutus.
Although Plutarch is his source, Shakespeare uses the information about these three men to weave a compelling story about Rome and the change that took place from a Republic to an Empire. He doesn't try to pass judgement but he presents the events.
In a history book, we can read about these events but Shakespeare puts a human face on the major players.
He also uses the play to ask important questions.
How does one stop Caesar from destroying the Republic since the people (mob) want to crown him king? Is the majority always right? Does a war, in this case a civil war, resolve anything? Does anybody win?
Shakespeare uses the information gained in Plutarch to spring board into these and other important questions.