I have to write an essay "critically analysing the leadership and legacy of Pompey the Great." Any helpful primary and secondary source recommendations would be greatly appreciated as I have to include historiography and don't want to rely completely on articles i've found on the internet that, whilst relevant, are not written by anyone highly regarded amongst historians.
So this period is the most documented in Roman history. The primary sources are vast. Plutarch's Life of Pompey's already been mentioned, but Pompey comes up in several Lives. Cicero discusses Pompey in numerous letters, and gave several speeches specifically addressing Pompey (De Imperio, for instance). Caesar wrote a history of his war against Pompey. After that, the imperial historians chime in - Appian's Civil Wars, Cassius Dio, the fragments of Diodorus Siculus. All of these are free in English on the internet if you look for them. You could make a career of studying the primary sources for Pompey.
And then there's the legacy. His son Sextus was a pain in the ass for Octavian for quite a while. He gets a good reception in Virgil, and I'm sure there's more out there. Depending how far you want to take it you might get as far as Shakespeare.
Good luck!
Depending on what your topic will be, I'd start with the classics, and analyze "The Life of Pompey" by Plutarch. While it is a moral biography and should be taken with more than a grain of salt, it is still insightful. Here is a link to the work in HTML at Project Gutenberg: Life of Pompeius
Also, for secondary sources, I find Chester G. Starr's "A History of the Ancient World" useful for a historical overview of Civil War era Roman history.