Evidently the lawyer's only activities after the first year of confinement were reading and thinking. His reading during the first year suggests that he was only using books as a way of killing time. Many people do this all their lives. Much of the literature that gets published is trash....
Much of the material in magazines is only there to fill up pages. Romances and mysteries are especially popular. They are escapist reading. The books are often formulaic, and after a while an intelligent reader will realize that they are reading the same plots with different characters and different settings. The reader gets little out of them except temporary diversion.
The lawyer's taste changes because he is in a serious situation. He has to spend fifteen years in isolation. He naturally becomes more serious in his thinking, and this leads to his becoming more serious in his tastes in reading. Like a lot of professionals, he may be realizing that he is woefully ignorant about almost everything except his speciality, which happens to be the law. The banker cannot really know what is going on in the lawyer's mind, and neither can the reader. This can only be guessed at from the books the prisoner requests. Books become a big part of his life because they can serve as companions, acquaintances, even friends. In other words, they are serving the lawyer as substitutes for interaction with real people. The classics offer better "company" than the works of hack writers whose main interest is in making money.
In the first year the books he sent for were principally of a light character; novels with a complicated love plot, sensational and fantastic stories, and so on. In the second year the piano was silent in the lodge, and the prisoner asked only for the classics.
Books offer opportunities for anyone to meet some of the wisest and most sincere and interesting people who ever lived. Chekhov himself is an example. Leo Tolstoy is an even better example. Holden Caulfield, the lonely boy who narrates The Catcher in the Rye, writes:
What really knocks me out is a book that, when you're all done reading it, you wish the author that wrote it was a terrific friend of yours and you could call him up on the phone whenever you felt like it.
The reader himself becomes a better and wiser person as a result of forming friendships with such intellectual and creative people. This was an unexpected benefit of the bet the lawyer made with the banker. The lawyer thought he could stand fifteen years of solitary confinement, but he didn't know how he was going to do it until he found himself all alone.