How does the author illustrate the lawyer's changes in "The Bet"?

At the beginning of the story, the young lawyer is bold, daring, determined and impulsive. Towards the end, he is a completely changed person. He has grown into a self-contented and calm middle aged man, who has no more desire for wealth or luxury.

This dramatic change in lawyer doesn’t occur in just a couple of weeks or months. It took him a whopping fifteen years of solitary confinement and lots and lots of reading to undergo such a drastic change. The author O. Henry presents a convincing account showing how gradually the change effected in him.

In the first year, he would play the piano day and night, but in the second year he stopped playing it. Unlike sending for novels with “complicated love plot, sensational and fantastic stories” in the first year, he asked “only for the classics” in the second year.

Again in the fifth year, music was heard again. He stopped reading and other activities as well. He would eat, drink and lie throughout the day. He would sometimes, speak angrily to himself. His frustration seemingly increased to a disturbing level during this period, as:

“Sometimes at night he would sit down to write; he would spend hours writing, and in the morning tear up all that he had written. More than once he could be heard crying.”

One of the most significant changes came in the sixth year. He started studying languages, history and philosophy. He became so engrossed in his pursuit of knowledge that in only four years, he completed studying “some six hundred volumes.”

We notice a drastic change in him when we read his letter written in six different languages. His following observation reveals that he is a much transformed man now:

“The geniuses of all ages and of all lands speak different languages, but the same flame burns in them all. Oh, if you only knew what unearthly happiness my soul feels now from being able to understand them!"

Having versed himself in different languages of the world and having read philosophy and history of the world, he now reads “nothing but the Gospel.” And then his focus shifted to theology and histories of religion.  

The final and the most unexpected change that we notice in him is when he relinquishes the two million rubles for which he had staked his most precious fifteen years of his life.

In this way the author paints a plausible picture of the lawyer's gradual spiritual development.

Why does the lawyer's book choices progress over the years in "The Bet"?

In Chekhov's short story, "The Bet," there is a progression in the books the lawyer reads because his interests change over the course of his confinement.

The lawyer begins by trying to distract himself from his captivity by light reading. This seems not to work, and for several years, he reads little or nothing. In the sixth year, however, the lawyer starts to read serious books, studying languages, philosophy, and history. This is because he has decided to make the best possible use of his confinement and improve his mind. Over the course of the next four years, he reads about 600 books.

In his tenth year, the lawyer reads the Gospels. It is clear that these have a profound effect on him, since he reads nothing else for nearly a year and then nothing but theological texts and books on the history of religion for the next two years. It is clear that the lawyer has progressed from serious study of multiple subjects to concentrating his mind exclusively on religious faith. Chekhov then notes,

In the last two years of his confinement the prisoner read an immense quantity of books quite indiscriminately. At one time he was busy with the natural sciences, then he would ask for Byron or Shakespeare.

In this final phase of the lawyer's reading, he is trying to find any reason he can to value this world. He reads as widely as he can in a desperate attempt to find anything that attracts him, from science to poetry. He finds nothing and, by the end of the story, has changed from a normal, worldly young man to an ascetic whose eyes are permanently fixed on heaven. Chekov's account of his reading shows how he reaches this point.