About The Republic (from Wikipedia):
The Republic (Greek: Πολιτεία, Politeia) is a Socratic dialogue, written by Plato around 380 BC, concerning the definition of justice and the order and character of the just city-state and the just man.
It is Plato’s best-known work and has proven to be one of the most intellectually and historically influential works of philosophy and political theory. In it, Socrates along with various Athenians and foreigners discuss the meaning of justice and examine whether or not the just man is happier than the unjust man by considering a series of different cities coming into existence “in speech”, culminating in a city ruled by philosopher-kings; and by examining the nature of existing regimes. The participants also discuss the theory of forms, the immortality of the soul, and the roles of the philosopher and of poetry in society.
About Timaeus (from Wikipedia):
Timaeus is one of Plato’s dialogues, mostly in the form of a long monologue given by the title character Timaeus of Locri, written c. 360 BC. The work puts forward speculation on the nature of the physical world and human beings and is followed by the dialogue Critias. In it is found Plato’s introductory descriptions of Atlantis.
About Critias (from Wikipedia):
Critias one of Plato’s late dialogues, recounts the story of the mighty island kingdom Atlantis and its attempt to conquer Athens, which failed due to the ordered society of the Athenians. Critias is the second of a projected trilogy of dialogues, preceded by Timaeus and followed by Hermocrates. The latter was possibly never written and Critias was left incomplete. Because of their resemblance (e.g. in terms of persons appearing), modern classicists occasionally combine both Timaeus and Critias as Timaeus-Critias.