Paradise Lost is a real-life 17th century English epic poem by John Milton. It tells the story of Adam and Eve, as well as how Satan came into being.
There are numerous parallels and shared motifs between Paradise Lost and the Fallout universe. Nuclear technology could be seen as an analog to the fruit of knowledge, the Old World to Eden, and so on. The quotes available on the terminal reinforce this.
The Speech bonus it provides may allude to Satan's ability in manipulation and rhetoric, which makes him seem sympathetic in spite of his flawed logic. Satan is considered one of the most convincing speakers in literature.
A curious contradiction arises 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 is Dante Alighieri's Inferno, the first canticle of his Divine Comedy.