Although I almost always find knowledge to be beneficial, in this particular case, reading Paradise Lost seems to set the creature on an even more vindictive and destructive path than he has been on thus far. Through this tale, he learns of Adam's plight, and he tells Victor, "Remember, that I am thy creature: I ought to be thy Adam." Through contrast, he sees what is possible between a Creator and His created when he reads Paradise Lost. He realizes the depth of nurturing and guidance that could exist, and he contrasts this with the vast neglect he has personally received from Victor. To compound this sense of loss, he finds notes Victor has written which detail "the minutest description of [his] odious and loathsome person ... in language which painted [Victor's] own horrors and rendered [the creature's] indelible."
The creature finds that this particular work "excited different and far deeper emotions" within himself than the other works he reads. It is in Paradise Lost that he also realizes that Adam is gifted with a companion to share his life with: Eve. It is likely from this work that the seed is planted for a desire for his own mate, and he approaches Victor with this new demand later in their conversation. Because Victor at first agrees to construct a female for the creature and then rips her to pieces before the creature's eyes, the creature sets out on a new spree of vindictive retribution which will claim the life of everyone Victor holds dear.
Although he is gifted with new understandings through reading Paradise Lost, the creature ultimately only suffers because of it. Therefore, I would argue that this book is detrimental to his already fragile mental health and only brings him misery as he finds himself more likened to Satan than to Adam.