Paradise Lost is a real-life 17th century English epic poem by John Milton. It retells the Judeo-Christian story of Adam and Eve, their temptaion by Satan, the commission of the original sin, and their expulsion from Eden. There are numerous parallels to be drawn between this story and the history of the Fallout universe - the technology of nuclear weaponry as the forbidden fruit, Vault-Tec as Satan, the Old World as Eden, the Old World nations as Adam and Eve, and the people of the wastes as their descendents. Themes of truth, falling, and redemption run throughout the game and the poem, and the Fallout series is rife with Biblical references.
The fact that the book provides a Speech increase may refer to Satan's expertise in manipulation and rhetoric.
A curious contradiction can be observed when talking to Tulip. She explains that Underworld is based on this book which she says is about a man making a trip through the Underworld. The book she is actually referring to and the one Underworld is based on is actually Dante Alighieri's Inferno, the first canticle of his Divine Comedy.