Epistulae ad Brutum (Letters to Brutus) is a collection of letters between Roman politician and orator Marcus Tullius Cicero and fellow politician, and conspirator against Julius Caesar, Marcus Junius Brutus.
 The letters in this collection, when combined with Cicero's other letters, are considered some of the most reliable sources of information for the period leading up to the fall of the Roman Republic.
 Cicero became acquainted with Brutus through his close friend Titus Pomponius Atticus, an admirer of Brutus.
 Their personal relationship likely grew during their time together in opposition to Caesar during the civil war in 49 BCE, it being firmly established by the time Cicero returned to Rome in the autumn of 47.
The personal life of Marcus Tullius Cicero provided the underpinnings of one of the most significant politicians of the Roman Republic.
 Cicero, a Roman statesman, lawyer, political theorist, philosopher, and Roman constitutionalist, played a critical role in the transformation of the Roman Republic into the Roman Empire.
 A contemporary of Julius Caesar, Cicero is widely considered one of Rome's greatest orators and prose stylists.
The political career of Marcus Tullius Cicero began in 75 BC with his election to the office of quaestor, and ended in 43 BC, when he was assassinated upon the orders of Mark Antony.
 Cicero, a Roman statesman, lawyer, political theorist, philosopher, and Roman constitutionalist, reached the height of Roman power, the Consulship, and played a critical role in the transformation of the Roman Republic into the Roman Empire.
 A contemporary of Julius Caesar, Cicero is widely considered one of Rome's greatest orators and prose stylists.
Matthias Gelzer (19 December 1886, Liestal – 23 July 1974, Frankfurt am Main) was a Swiss-German classical historian, known for his studies of the Roman Republic in regard to its politics and society.
 He was the author of highly regarded biographies on Julius Caesar, Pompey and Cicero.
Julia (c. 130 BC – 69 BC) was a daughter of Gaius Julius Caesar II (praetor-grandfather of Caesar) and Marcia (daughter of praetor Quintus Marcius Rex).
 She was a sister of Gaius Julius Caesar III (the father of Julius Caesar) and Sextus Julius Caesar III, consul in 91 BC.
Gaius Rabirius was a Roman senator who was involved in the death of Lucius Appuleius Saturninus in 100 BC.
 Titus Labienus, a Tribune of the Plebs whose uncle had lost his life among the followers of Saturninus on that occasion, was urged by fellow Senator and patron Julius Caesar to accuse Rabirius of participating in the murder.
 Caesar's real objective was to warn the Senate against interference by force with popular movements, to uphold the sovereignty of the people and the inviolability of the person of the tribunes, at the time of the conspiracy of Lucius Sergius Catilina.
 The obsolete accusation of "perduellio" was revived, and the case was heard before Caesar and his cousin Lucius Julius Caesar as commissioners specially appointed ("duumviri perduellionis").
 Rabirius was condemned, and the people, to whom the accused had exercised the right of appeal, were on the point of ratifying the decision, when Quintus Caecilius Metellus Celer pulled down the military flag from the Janiculum, which was equivalent to the dissolution of the assembly.
 Caesar's object having been attained, the matter was then allowed to drop.
 The defense was taken by Marcus Tullius Cicero, consul at the time; the speech is extant: "Pro Rabirio reo perduellionis".
Gaius Julius Caesar (Latin: CAIVS IVLIVS CAESAR, ] , born: 13 July 100 BC – 15 March 44 BC), usually called Julius Caesar, was a Roman politician and general who played a critical role in the events that led to the demise of the Roman Republic and the rise of the Roman Empire.
 He is also known as a notable author of Latin prose.
Marcus Tullius Cicero Minor ("Minor" Latin for "‘the younger’"), or Cicero the Younger, was born in 65 BC.
 He was the son of Marcus Tullius Cicero, who as a distinguished orator and consular senator was one of the leading figures of the Roman Republic during the 1st century BC, and his first wife, Terentia.
 Cicero minor had an elder sister, Tullia, who was born in 79 BC and died in 45 BC.
 In the beginning Cicero wished to have a military career.
 On the outbreak of civil war in 49 BC, he joined the side of Pompey like his father.
 After Pompey’s defeat by Julius Caesar at Pharsalus in 48 BC, Cicero minor was pardoned by Caesar.
The Julii Caesares were the most illustrious family of the patrician "gens Julia".
 The family first appears in history during the Second Punic War, when Sextus Julius Caesar was praetor in Sicily.
 His son, Sextus Julius Caesar, obtained the consulship in 157 BC; but the most famous descendant of this stirps is Gaius Julius Caesar, a general who conquered Gaul and became the undisputed master of Rome following the Civil War.
 Having been granted dictatorial power by the Roman Senate and instituting a number of political and social reforms, he was assassinated in 44 BC.
 After overcoming several rivals, Caesar's adopted son and heir, Gaius Julius Caesar Octavianus, was proclaimed Augustus by the senate, inaugurating what became the Julio-Claudian line of Roman emperors.
The writings of Marcus Tullius Cicero constitute one of the most famous bodies of historical and philosophical work in all of classical antiquity.
 Cicero, a Roman statesman, lawyer, political theorist, philosopher, and Roman constitutionalist, lived from 106 to 43 BC.
 He was a Roman senator and consul (chief-magistrate) who played a critical role in the transformation of the Roman Republic into the Roman Empire.
 A contemporary of Julius Caesar, Cicero is widely considered one of Rome's greatest orators and prose stylists.

Question: Which important Roman figure did Matthias Gelzer write a biography for, besides Julius Caesar and Cicero?
Answer: Pompey