An epic poem about the Trojan War, comprised of 24 books (about 600 pages), written about 700 BC, it is the oldest piece of western and Greek literature. The Iliad as well as the Odyssey were written by Homer, supposedly a blind poet, but the poems had a long oral history before it was written down, and academics debate about how they transitioned from oral transmission to being written. Many scholars believe it is a mythologized version of a real Trojan War between the Greeks and Troy that happened several centuries before, especially after the ruins of Troy were discovered.
The Iliad begins nine years into the Trojan War, the hero Achilles (the greatest warrior in the Greek Army) got upset at his commander Agamemnon after a quarrel over a slave girl, and Achilles decided to withdraw from the war effort. The reason that the Greeks wanted to invade Troy was to rescue Helen, a Greek noblewoman who was wed to Menelaus, but was captured by the Trojans. Most of the poem is dialogue; I got through about 150 pages (about 1/4 of the poem) before I got bored of it.
The gods of Greek mythology play a large role in the plot, many humans are descendants of the gods, who reside on a sort of alternate universe on Mount Olympus but frequently come down to intervene on human affairs. The Iliad contains detailed descriptions of bloody battles, interestingly it likes to introduce a bit of story about each of the hundreds of minor characters, then immediately describe how they die a grisly death at the hands of a Trojan or Greek warrior.