We shouldn't trust Lucan's Pharsalia for history, as it is decidedly not so. Besides being far removed (by almost a century!), Lucan had a clear ax to grind:
Though the poem covered historical events, Lucan did not mean to convey a historically accurate poem. First, it must be remembered that Caesar's death was over 90 years prior the composition of the poem's first book. Second, it was more important for Lucan to convey Stoic principles. which did not easily line up with a the Julio-Claudian will, as well as deliberate invert the pro-Caesar, pro-Augustan point-of-view he found in the Aeneid.
Cassius Dio, Appian, and Suetonius are all mentioned and worth reading. We also have Plutarch's Life of Caesar, Life of Pompey, and Life of Cato the Younger, as well as Life of Cicero, all of which have useful information in them and can be found on Lacus Curtius.
You also have the three additional works appended to the end of Caesar's Commentarii: the Bellum Alexandrinum, Bellum Africum, and Bellum Hispaniense, which were probably written subordinates of Caesar perhaps even after his death. (The Bellum Alexandrinum is thought to be written by either Oppius or Hirtius, the other two are wholly anonymous.)
Also do not neglect Cicero's letters, which apart from Caesar's works provide the best first-hand account of the lead up and consequences of the war.
Finally, though fragmentary, Nicholas of Damascus' Life of Augustus contains some information, though the work is about Augustus and therefore incidental to the war.