As seen in Frankenstein, how do Plutarch's Lives, Goethe's Sorrows of Werter, and Milton's Paradise Lost influence the creature?
The books the creature, in Mary Shelley's Frankenstein, finds (Lives, Sorrows of Werter, and Paradise Lost) greatly influence the creature. In chapter fifteen of the novel, the creature defines the impact of the books.
They produced in me an infinity of new images and feelings that sometimes raised me to ecstasy, but more frequently sunk me into the lowest dejection.
This ext defined the history of the world for the creature. It was this text which allowed the creature to be introduced to "high thoughts." The text allowed the creature to think about others, outside of himself:
He elevated me above the wretched sphere of my own reflections to admire and love the heroes of past ages.
Goethe's Sorrows of Werter
This text allowed the creature to examine the simpler things in life. It opened his eyes to the domestic sphere and the pain associated with life.
The disquisitionsupon death and suicide were calculated to fill me with wonder. I did not pretend to enter into the merits of the case, yet I inclined towards the opinions of the hero, whose extinction I wept, without precisely understanding it.
That said, although the text opened the creature to different feelings, he still states that the text is above him.
Milton's Paradise Lost
Paradise Lost was the text which most impacted the creature. It was through this text that the creature first realizes how his "father" abandoned him, like God abandoned Satan. This text allows the creature to feel a connection with another being--something he has lacked to this point in his "life." The creature read the text "as a true history," as he read the other texts. He believed the text to be historical and factual in nature. Essentially, the most important idea behind the text was the creature's engagement with another being--given to this point he has felt utterly alone.
Like Adam, I was apparently united by no link to any other being in existence; but his state was far different from mine in every other respect...Many times I considered Satan as the fitter emblem of my condition; for often, like him, when I viewed the bliss of my protectors, the bitter gall of envy rose within me.
In the end, all of the texts opened the creautre's eyes to the immediate world around him. Limited by his "age," the creature lacked basic knowledge about the world, although he was expected to live in it alone. Through the texts, the creature is able to discover certain things about the world and find his place in it--through his relation and understanding.
After Victor has abandoned his creature, the creature spends some time observing the DeLacey family and learning language from them. Once he has done so, he is able to read Plutarch's Lives, The Sorrows of Werther, and Paradise Lost, along with Victor's journal, in which the creature reads about his creation and Victor's feelings of horror at his work. Each of these texts has an impact on the creature and helps form his characteristics and motives.
Plutarch's Lives is a text about great figures of the ancient world. This teaches the creature what makes a man significant and what honorable qualities are valued by the society around him. Remember that the creature has had no education from his creator and is left alone to discover what he can about the world into which he has been thrust. The creature aspires to these heights he reads about in the Lives, and as we can see from his narration, he is not inherently evil and does want to be good.
The Sorrows of Werther is a sentimental novel, so it allows the creature to tap into his emotional side and to learn empathy. This is important because it shows us that the creature has feelings and can be affected greatly by the emotions of others. Again, he is not evil and is actually a tender-hearted being.
Paradise Lost is the one of these texts that has the most significant impact on the creature. He reads the story of God's creation of Adam and contrasts it with his experience with Victor, his creator. He feels more like Satan than like Adam, and this is a direct result of Victor's abandonment of and disgust with his creature. From the poem, the creature also learns the story of Adam and Eve, and this (along with his observation of the DeLacey clan) plants the idea in his mind that he could be happy if he had a female partner. He appeals to Victor to grant this wish, and Victor at first agrees but then goes back on his promise and destroys what he had completed of the female creature.
Frankenstein's journal is not listed in your question, but it is a fourth text that influences the creature greatly. He could have read the other texts with some sense of optimism if it hadn't been for what he learned about his creation and Victor's true feelings in the journal. However, he still thinks Victor can be won over if he can appeal to Victor's sympathy. Ultimately, the journal and Victor's destruction of the female creature combine to motivate the creature to take his revenge on Victor.