What would you do if you were in solitary confinement but had access to whatever physical objects you wanted (and, um, if TV hadn't been invented yet)? The first thing the pops into Shmoop's mind is pretty much the same thing that popped into the lawyer's mind—books. Duh. The banker totally gets it—obviously you need something to entertain you. But the longer the lawyer stays, the more the banker's idea of what books are and what they're for changes.
The banker sees the many, many books that the prisoner demands as a way for the lawyer to pass the time. And at first that does really seem like what he's all about, what with him asking for books with "light character; novels with a complicated love plot, sensational and fantastic stories" (1.16). The guy is lonely and sad and just wants an escape from his situation. We've all been there and we can totally get it. It's called beach reading.
Even when he turns from entertainment to study, the books still make sense. The lawyer "began zealously studying languages, philosophy, and history. He threw himself eagerly into these studies […] some six hundred volumes were procured at his request" (1.17). Now, Shmoop doesn't know if you're planning on grad school, but take it from us—this kind of studying is a slightly beefed up PhD course load. So, you know, it's still within the realm of what regular humans do. And the next phase is also a pretty understandable one—the lawyer turns to religion, reading the Bible and theology, because hey, why not?
But then we suddenly see the lawyer veering off the deep end. Instead of continuing his studies or entertainment, he starts reading "an immense quantity of books quite indiscriminately" (1.20). Science, novels, medicine, philosophy, poetry—just a totally strange mix of stuff. To the banker, this "suggested a man swimming in the sea among the wreckage of his ship, and trying to save his life by greedily clutching first at one spar and then at another" (1.20). Clearly, something is changing in the way the lawyer is relating to the books.
And then, in his letter, we finally get a sense of what's happened:
"For fifteen years I have been intently studying earthly life. It is true I have not seen the earth nor men, but in your books I have drunk fragrant wine, I have sung songs, I have hunted stags and wild boars in the forests, have loved women. . . . […] In your books I have flung myself into the bottomless pit, performed miracles, slain, burned towns, preached new religions, conquered whole kingdoms. . . . Your books have given me wisdom. All that the unresting thought of man has created in the ages is compressed into a small compass in my brain. I know that I am wiser than all of you. And I despise your books, I despise wisdom and the blessings of this world." (2.14-16)
The lawyer no longer perceives the books as some external thing. Instead, he has "compressed" so much of the "unresting thought of man" into his brain that he is confusing knowledge with experience. He doesn't get that he has only read about "drinking wine, loving women, burning towns," and so on. Instead, he actually feels like he himself has lived and felt these things. Humans crave bodily interaction, and since he's been deprived of an actual life, his brain has created a substitute one for him.