Freak reads a lot of books for a couple of reasons. First of all, he is highly intelligent. He has an insatiable thirst for knowledge in all fields, and is fascinated with what he learns from books. Freak is especially interested in literature, as evidenced by his fascination for the Tales of King Arthur, and biology and mechanics, especially as concerns the discipline of robotics. Freak holds onto a frail hope that the scientific world will come up with a way to provide humans with artificial body parts before he himself succumbs to his debilitating disease.
Another reason why Freak reads a lot is because, for him, it is a form of recreation. His physical disability prevents him from engaging in activities that other children enjoy, such as organized sports, for example. Although Freak can get around on his own, his size and limb weakness makes it difficult, if not impossible, for him to keep up with his peers. Until he meets Max and the two transform themselves into "Freak the Mighty," with Freak riding everywhere on Max's shoulders, he most likely spends a lot of time alone, and amuses himself by reading. Freak has a vivid imagination which cannot be contained, and in the books he reads, he vicariously lives the adventures he is not able to experience on his own. His favorites are, again, the Tales of King Arthur, and through the combination of his reading and imagination, he puts himself in the position of being an explorer and hero, pursuing and vanquishing monstrous dragons in a self-created world which is kinder to him than reality.
For Freak, reading represents escape; escape from his physical limitations, escape from his current life scenario, and escape from his literal environmental surroundings. Freak likes to "become" the characters in his books, as doing so allows him to seem more capable than he truly is in real life.
This escapism also appeals to Freak's intelligent side. He is a very bright boy, and reading empowers him. This empowerment is a great contrast to the constraints of his everyday life, dealing with physical handicaps and social alienation.